Color electrophoretic displays using same polarity reversing address pulse

ABSTRACT

An electrophoretic display comprising a fluid including a first species of particles and a charge control agent disposed between first and second electrodes. When a first addressing impulse have an electrical polarity is applied to the medium, the first species of particles move in one direction relative to the electric field, but when a second addressing impulse, larger than the first addressing impulse but having the same electrical polarity, is applied to the medium, the first species of particles move in the opposed direction relative to the electric field.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/606,058, filed May 26, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/277,107, filed May 14, 2014, now U.S. Pat. No.9,697,778, which claimed the benefit of U.S. Provisional ApplicationSer. No. 61/823,031, filed May 14, 2013. The entire contents of theaforementioned copending application and of all U.S. patents andpublished and copending applications mentioned below are hereinincorporated by reference.

BACKGROUND OF INVENTION

This invention relates to colored electrophoretic displays, and morespecifically to electrophoretic displays capable of rendering more thantwo colors using a single layer of electrophoretic material comprising aplurality of colored particles.

The term color as used herein includes black and white. White particlesare often of the light scattering type.

The term gray state is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate gray state would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms black and white may be used hereinafterto refer to the two extreme optical states of a display, and should beunderstood as normally including extreme optical states which are notstrictly black and white, for example the aforementioned white and darkblue states.

The terms bistable and bistability are used herein in their conventionalmeaning in the art to refer to displays comprising display elementshaving first and second display states differing in at least one opticalproperty, and such that after any given element has been driven, bymeans of an addressing pulse of finite duration, to assume either itsfirst or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called multi-stable rather than bistable,although for convenience the term bistable may be used herein to coverboth bistable and multi-stable displays.

The term impulse, when used to refer to driving an electrophoreticdisplay, is used herein to refer to the integral of the applied voltagewith respect to time during the period in which the display is driven.

A particle that absorbs, scatters, or reflects light, either in a broadband or at selected wavelengths, is referred to herein as a colored orpigment particle. Various materials other than pigments (in the strictsense of that term as meaning insoluble colored materials) that absorbor reflect light, such as dyes or photonic crystals, etc., may also beused in the electrophoretic media and displays of the present invention.

Particle-based electrophoretic displays have been the subject of intenseresearch and development for a number of years. In such displays, aplurality of charged particles (sometimes referred to as pigmentparticles) move through a fluid under the influence of an electricfield. Electrophoretic displays can have attributes of good brightnessand contrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., Electrical toner movement for electronicpaper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., etal., Toner display using insulative particles charged triboelectrically,IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and7,236,291. Such gas-based electrophoretic media appear to be susceptibleto the same types of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous suspendingfluids as compared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155;        7,667,684; 7,791,789; 7,956,841; 8,040,594; 8,054,526;        8,098,418; 8,213,076; and 8,363,299; and U.S. Patent        Applications Publication Nos. 2004/0263947; 2007/0109219;        2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970;        2009/0004442; 2009/0225398; 2010/0103502; 2010/0156780;        2011/0164307; 2011/0195629; 2011/0310461; 2012/0008188;        2012/0019898; 2012/0075687; 2012/0081779; 2012/0134009;        2012/0182597; 2012/0212462; 2012/0157269; and 2012/0326957;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997;        6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600;        7,023,420; 7,034,783; 7,116,466; 7,119,772; 7,193,625;        7,202,847; 7,259,744; 7,304,787; 7,312,794; 7,327,511;        7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251;        7,602,374; 7,612,760; 7,679,599; 7,688,297; 7,729,039;        7,733,311; 7,733,335; 7,787,169; 7,952,557; 7,956,841;        7,999,787; 8,077,141; 8,125,501; 8,139,050; 8,174,490;        8,289,250; 8,300,006; and 8,314,784; and U.S. Patent        Applications Publication Nos. 2003/0102858; 2005/0122284;        2005/0179642; 2005/0253777; 2007/0091418; 2007/0103427;        2008/0024429; 2008/0024482; 2008/0136774; 2008/0150888;        2008/0291129; 2009/0174651; 2009/0179923; 2009/0195568;        2009/0322721; 2010/0045592; 2010/0220121; 2010/0220122;        2010/0265561; 2011/0187684; 2011/0193840; 2011/0193841;        2011/0199671; and 2011/0285754 (these patents and applications        may hereinafter be referred to as the MEDEOD (MEthods for        Driving Electro-optic Displays) applications);    -   (g) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent        Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,866,760. Accordingly, for purposes of the presentapplication, such polymer-dispersed electrophoretic media are regardedas sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called microcellelectrophoretic display. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called shutter mode in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode can beused in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word printing is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called front plane laminate(FPL) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term light-transmissive isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term light-transmissive should of coursebe interpreted to refer to transmission of the relevant non-visiblewavelengths. The substrate will typically be a polymeric film, and willnormally have a thickness in the range of about 1 to about 25 mil (25 to634 μm), preferably about 2 to about 10 mil (51 to 254 μm). Theelectrically-conductive layer is conveniently a thin metal or metaloxide layer of, for example, aluminum or ITO, or may be a conductivepolymer. Poly(ethylene terephthalate) (PET) films coated with aluminumor ITO are available commercially, for example as aluminized Mylar(Mylar is a Registered Trade Mark) from E.I. du Pont de Nemours &Company, Wilmington Del., and such commercial materials may be used withgood results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called double release sheet whichis essentially a simplified version of the front plane laminate of theaforementioned U.S. Pat. No. 6,982,178. One form of the double releasesheet comprises a layer of a solid electro-optic medium sandwichedbetween two adhesive layers, one or both of the adhesive layers beingcovered by a release sheet. Another form of the double release sheetcomprises a layer of a solid electro-optic medium sandwiched between tworelease sheets. Both forms of the double release film are intended foruse in a process generally similar to the process for assembling anelectro-optic display from a front plane laminate already described, butinvolving two separate laminations; typically, in a first lamination thedouble release sheet is laminated to a front electrode to form a frontsub-assembly, and then in a second lamination the front sub-assembly islaminated to a backplane to form the final display, although the orderof these two laminations could be reversed if desired.

U.S. Pat. No. 7,839,564 describes a so-called inverted front planelaminate, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

As indicated above most simple prior art electrophoretic mediaessentially display only two colors. Such electrophoretic media eitheruse a single type of electrophoretic particle having a first color in acolored fluid having a second, different color (in which case, the firstcolor is displayed when the particles lie adjacent the viewing surfaceof the display and the second color is displayed when the particles arespaced from the viewing surface), or first and second types ofelectrophoretic particles having differing first and second colors in anuncolored fluid (in which case, the first color is displayed when thefirst type of particles lie adjacent the viewing surface of the displayand the second color is displayed when the second type of particles lieadjacent the viewing surface). Typically the two colors are black andwhite. If a full color display is desired, a color filter array may bedeposited over the viewing surface of the monochrome (black and white)display. Displays with color filter arrays rely on area sharing andcolor blending to create color stimuli. The available display area isshared between three or four primary colors such as red/green/blue (RGB)or red/green/blue/white (RGBW), and the filters can be arranged inone-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Otherchoices of primary colors or more than three primaries are also known inthe art. The three (in the case of RGB displays) or four (in the case ofRGBW displays) sub-pixels are chosen small enough so that at theintended viewing distance they visually blend together to a single pixelwith a uniform color stimulus (‘color blending’). The inherentdisadvantage of area sharing is that the colorants are always present,and colors can only be modulated by switching the corresponding pixelsof the underlying monochrome display to white or black (switching thecorresponding primary colors on or off). For example, in an ideal RGBWdisplay, each of the red, green, blue and white primaries occupy onefourth of the display area (one sub-pixel out of four), with the whitesub-pixel being as bright as the underlying monochrome display white,and each of the colored sub-pixels being no lighter than one third ofthe monochrome display white. The brightness of the white color shown bythe display as a whole cannot be more than one half of the brightness ofthe white sub-pixel (white areas of the display are produced bydisplaying the one white sub-pixel out of each four, plus each coloredsub-pixel in its colored form being equivalent to one third of a whitesub-pixel, so the three colored sub-pixels combined contribute no morethan the one white sub-pixel). The brightness and saturation of colorsis lowered by area-sharing with color pixels switched to black. Areasharing is especially problematic when mixing yellow because it islighter than any other color of equal brightness, and saturated yellowis almost as bright as white. Switching the blue pixels (one fourth ofthe display area) to black makes the yellow too dark.

Multilayer, stacked electrophoretic displays are known in the art; J.Heikenfeld, P. Drzaic, J-S Yeo and T. Koch, Journal of the SID, 19(2),2011, pp. 129-156. In such displays, ambient light passes through imagesin each of the three subtractive primary colors, in precise analogy withconventional color printing. U.S. Pat. No. 6,727,873 describes a stackedelectrophoretic display in which three layers of switchable cells areplaced over a reflective background. Similar displays are known in whichcolored particles are moved laterally (see International Application No.WO 2008/065605) or, using a combination of vertical and lateral motion,sequestered into micropits. In both cases, each layer is provided withelectrodes that serve to concentrate or disperse the colored particleson a pixel-by-pixel basis, so that each of the three layers requires alayer of thin-film transistors (TFT's) (two of the three layers of TFT'smust be substantially transparent) and a light-transmissivecounter-electrode. Such a complex arrangement of electrodes is costly tomanufacture, and in the present state of the art it is difficult toprovide an adequately transparent plane of pixel electrodes, especiallyas the white state of the display must be viewed through several layersof electrodes. Multi-layer displays also suffer from parallax problemsas the thickness of the display stack approaches or exceeds the pixelsize.

U.S. Applications Publication Nos. 2012/0008188 and 2012/0134009describe multicolor electrophoretic displays having a single back planecomprising independently addressable pixel electrodes and a common,light-transmissive front electrode. Between the back plane and the frontelectrode is disposed a plurality of electrophoretic layers. Displaysdescribed in these applications are capable of rendering any of theprimary colors (red, green, blue, cyan, magenta, yellow, white andblack) at any pixel location. However, there are disadvantages to theuse of multiple electrophoretic layers located between a single set ofaddressing electrodes. The electric field experienced by the particlesin a particular layer is lower than would be the case for a singleelectrophoretic layer addressed with the same voltage. In addition,optical losses in an electrophoretic layer closest to the viewingsurface (for example, caused by light scattering or unwanted absorption)may affect the appearance of images formed in underlying electrophoreticlayers.

Attempts have been made to provide full-color electrophoretic displaysusing a single electrophoretic layer. See, for example, U.S. PatentApplication Publication No. 2011/0134506. However, in the current stateof the art such displays typically involve compromises such as slowswitching speeds (as long as several seconds) or high addressingvoltages.

The present invention seeks to provide a color display using only asingle electrophoretic layer but capable of displaying more than two,and preferably all colors at every location of the active area of thedisplay, and a method of driving such an electrophoretic display.

SUMMARY OF INVENTION

Accordingly, this invention provides an electrophoretic mediumcomprising a fluid and at least a first species of particles disposed inthe fluid, the first species of particles being such that when a firstaddressing impulse is applied to the medium, the first species ofparticles move in one direction relative to the electric field, but whena second addressing impulse, larger than the first addressing impulsebut having the same polarity, is applied to the medium, the firstspecies of particles move in the opposed direction relative to theelectric field.

The first and second addressing impulses may differ from each other infield strength, duration or both. Furthermore, although the firstaddressing impulse comprises applying a first electric field is appliedto the medium for a first period and that the second addressing impulsecomprises applying a second electric field is applied to the medium fora second period, it is not intended to imply that the first and secondelectric fields must be constant over the first and second periodsrespectively, nor is it to be understood that the first and secondelectric field necessarily differ in magnitude from each other or thatthe first and second periods differ in duration. It is only requiredthat the second addressing impulse (i.e., the integral with respect totime of the voltage used to create the second electric field, taken overthe second period) be greater than the first addressing impulse.

This invention also provides a method of driving an electrophoreticmedium comprising a fluid and at least a first species of particlesdisposed in the fluid, the method comprising:

-   -   (a) applying a first addressing impulse to the medium, thereby        causing the first species of particles to move in one direction        relative to the electric field; and    -   (b) applying a second addressing impulse, larger than the first        addressing impulse but having the same polarity, to the medium,        thereby causing the first species of particles to move in the        opposed direction relative to the electric field.

This invention also provides an electrophoretic display capable ofrendering multiple different colors, the display comprising anelectrophoretic medium comprising a fluid and a plurality of particlesdisposed in the fluid, the display further comprising first and secondelectrodes disposed on opposed sides of the electrophoretic medium,wherein upon application of a first addressing impulse to theelectrophoretic medium the particles move towards the first electrode,but upon application of a second addressing impulse, larger than but ofthe same polarity as the first addressing impulse, the particles movetowards the second electrode.

In one form of such an electrophoretic display, upon application of thefirst addressing impulse the particles move towards the more positiveelectrode but upon application of the second addressing impulse theparticles move towards the more negative electrode. In such anelectrophoretic display, the particles will normally have a negativecharge when no electric field is being applied to the particles. Such adisplay may further comprise a second type of particles which have acolor different from the first type of particles and which move towardsthe more negative electrode upon application of either the first orsecond addressing impulse.

The aforementioned media and displays of the present invention mayhereinafter for convenience be referred to as the charge-switchingparticles or CSP media and displays of the invention.

In another aspect, this invention provides an electrophoretic mediumcomprising a fluid and first, second and third species of particlesdisposed in the fluid. The first species of particles bear charges ofone polarity, while the second and third species of particles bearcharges of the opposite polarity. The characteristics of the first,second and third species of particles are such that theparticle-particle interactions are less between the particles of thefirst species and the particles of the second species than between theparticles of the first species and the particles of the third species.When a first addressing impulse is applied to the electrophoreticmedium, the first and third species of particles move in one directionrelative to the electric field and the second species of particles movein the opposed direction relative to the electric field. When a secondaddressing impulse, larger than the first addressing impulse but of thesame polarity is applied to the electrophoretic medium, the firstspecies of particles move in said one direction relative to the electricfield, while the second and third species of particles move in saidopposed direction relative to the electric field.

In such electrophoretic medium, one way of controlling the interactionsamong the first, second and third species of particles is by controllingthe type, amount and thickness of polymeric coatings on the particles.For example, to control the particle characteristics such that theparticle-particle interactions are less between the particles of thefirst species and the particles of the second species than between theparticles of the first species and the particles of the third species,the second species of particles may bear a polymeric surface treatment,and the third species of particles may either bearing no polymericsurface treatment or bearing a polymeric surface treatment having alower mass coverage per unit area of the particle surface than thesecond species of particles. More generally, the Hamaker constant (whichis a measure of the strength of the Van der Waals interaction betweentwo particles, the pair potential being proportional to the Hamakerconstant and inversely proportional to the sixth power of the distancebetween the two particles) and/or the interparticle spacing need(s) tobe adjusted by judicious choice of the polymeric coating(s) on the threespecies of particles.

In another aspect, this invention provides an electrophoretic displaycapable of rendering multiple different colors, the display comprisingan electrophoretic medium and first and second electrodes disposed onopposed sides of the electrophoretic medium. The electrophoretic mediumcomprises a fluid and a plurality of a first species of particles havinga negative charge, a plurality a second species of particles having apositive charge, and a plurality of a third species of particles havinga positive charge. The particle pair interactions, both Coulombic andattractive non-Coulombic, are less between the first species ofparticles and the second species of particles than between the firstspecies of particles and the third species of particles. With a firstaddressing impulse the particles of the first and third species movetowards the more positive electrode and the particles of the secondspecies move towards the more negative electrode. However, with a secondaddressing impulse larger than the first addressing impulse, theparticles of the first species move towards the more positive electrodeor remain in the vicinity of the more positive electrode and theparticles of the third species move towards the more negative electrode,while the particles of the second species remain in the vicinity of themore negative electrode.

For reasons which will appear below, these electrophoretic media anddisplays of the present invention may hereinafter for convenience bereferred to as the spot color or SC media and displays of the invention.

In another aspect, this invention provides an electrophoretic mediumcomprising a fluid and first, second and third species of particlesdisposed in the fluid. The fluid is dyed a first color. The firstspecies of particles are light-scattering, and bear charges of onepolarity, while the second and third species of particles are non-lightscattering, are of second and third colors respectively different fromthe first color and from each other, and bear charges of the oppositepolarity. The characteristics of the first, second and third species ofparticles are such that the particle-particle interactions are lessbetween the particles of the first species and the particles of thesecond species than between the particles of the first species and theparticles of the third species. When a first addressing impulse isapplied to the electrophoretic medium, the first and third species ofparticles move in one direction relative to the electric field and thesecond species of particles move in the opposed direction relative tothe electric field. When a second addressing impulse, larger than thefirst addressing impulse but of the same polarity is applied to theelectrophoretic medium, the first species of particles move in said onedirection relative to the electric field, while the second and thirdspecies of particles move in said opposed direction relative to theelectric field. When a third addressing impulse, larger than the secondaddressing impulse but of the same polarity is applied to theelectrophoretic medium, the first species of particles move in saidopposed direction relative to the electric field, while the second andthird species of particles continue to move in said opposed directionrelative to the electric field.

This invention also provides an electrophoretic display capable ofrendering multiple different colors, the display comprising anelectrophoretic medium and first and second electrodes disposed onopposed sides of the electrophoretic medium. The electrophoretic mediumcomprises a fluid dyed a first color; a plurality of a first species oflight-scattering particles having a negative charge; a plurality of asecond species of non-light scattering particles having a second colorand a positive charge; and a plurality of a third species ofnon-light-scattering particles having a third color and a positivecharge. The particle pair interactions (which may be adjusted in waysdescribed above in relation to the SC media and displays of the presentinvention), both Coulombic and attractive non-Coulombic, are lessbetween the first species of particles and the second species ofparticles than between the first species of particles and the thirdspecies of particles. When a first addressing impulse is applied to thedisplay, the first and third species of particles move towards the morepositive electrode and the pigment particles of the second type movetowards the more negative electrode. When a second addressing impulse,larger than the first addressing impulse, is applied to the display, thefirst species of particles move towards the more positive electrode orremain in the vicinity of the more positive electrode and the thirdspecies of particles move towards the more negative electrode, while thesecond species of particles remain in the vicinity of the more negativeelectrode. When a third addressing impulse, larger than the secondaddressing impulse, is applied to the display, the first species ofparticles move towards the more negative electrode.

For reasons which will appear below, these electrophoretic media anddisplays of the present invention may hereinafter for convenience bereferred to as the full color or FC media and displays of the invention.

Finally, the present invention provides an electrophoretic mediumcomprising a fluid and at least one type of charged particle disposed inthe fluid and capable of moving through the fluid when an electric fieldis applied to the medium, the medium further comprising a charge-controladjuvant capable of imparting a more positive charge to the chargedparticles, wherein the charge-control adjuvant is a metal salt of acarboxylic acid, wherein the metal is chosen from the group consistingof lithium, magnesium, calcium, strontium, rubidium, barium, zinc,copper, tin, titanium, manganese, iron, vanadium, and aluminum.

This invention extends to a front plane laminate, double release sheet,inverted front plane laminate or electrophoretic display comprising anelectrophoretic medium of the present invention. The displays of thepresent invention may be used in any application in which prior artelectro-optic displays have been used. Thus, for example, the presentdisplays may be used in electronic book readers, portable computers,tablet computers, cellular telephones, smart cards, signs, watches,shelf labels and flash drives.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 of the accompanying drawings is a schematic cross-section throughan electrophoretic display of the present invention.

FIG. 2 is a highly schematic cross-section through an electrophoreticdisplay having two non-blocked electrodes and shows the displacement ofcharged species within the electrophoretic fluid during switching.

FIGS. 3A and 3B are graphs illustrating the electrical potential withinan electrophoretic medium as a function of distance in the direction ofan applied electric field.

FIG. 4 is a highly schematic cross-section, similar to that of FIG. 2,through an electrophoretic display having two non-blocked electrodes andshows the displacement of charged species during switching with anadditional flow of electrochemically-generated ions.

FIGS. 5A and 5B are schematic cross-sections, not to scale, through anegatively charge electrophoretic particle and surrounding fluid, andillustrate a hypothesis concerning the mode of transport of charge inthe vicinity of the particle.

FIG. 6 shows micrographs of illustrating the movement of anegatively-charged white pigment in certain experiments described below.

FIG. 7 is a highly schematic cross-section, similar to those of FIGS. 2and 4, through an electrophoretic display and shows the displacement ofcharged species during switching with an additional flow ofelectrochemically-generated ions.

FIGS. 8A and 8B are schematic cross-sections through an encapsulated CSPelectrophoretic display of the present invention showing the variousoptical states of the display under first and second addressingimpulses.

FIG. 9 is a graph showing the colors which can be produced by a CSPelectrophoretic display of the present invention, such as that shown inFIGS. 8A and 8B.

FIGS. 10A and 10B are schematic cross-sections through an encapsulatedSC electrophoretic display of the present invention showing the variousoptical states of the display under first and second addressingimpulses.

FIGS. 11A-11D are schematic cross-sections through an encapsulated FCelectrophoretic display of the present invention showing the variousoptical states of the display under first, second, third and oppositepolarity addressing impulses.

FIG. 12 is a graph illustrating the effect of adding minor proportionsof aluminum di(t-butyl)salicylate as a charge control agent control ofcharging of pigments in electrophoretic fluids of the present invention;and

FIGS. 13A and 13B are L*b* graphs showing the colors obtained fromcertain displays of the present invention, as described in Example 2below.

FIG. 14 is a graph showing the driving waveform applied to a display ofthe present invention in Example 3 below, and the resulting L* and b*values.

FIGS. 15A-15J are graphs showing various waveforms used in theexperiments described in Example 6 below.

FIGS. 16A-16J are graphs showing the L*, a* and b* values as a functionof time obtained using the waveforms of FIGS. 15A-15J respectively.

FIG. 17 is a plot of the a*b* plane of conventional L*a*b* color spaceshowing the gamut of colors obtained in the experiments described inExample 6 below.

FIGS. 18 and 19 are plots of the a*b* plane similar to that of FIG. 17but showing the gamut of colors obtained in the experiments described inExamples 7 and 8 respectively below.

FIG. 20 is a graph similar to that of FIG. 14 but showing the resultsachieved with a modified waveform applied to a display of the presentinvention in Example 9 below.

FIG. 21 is a diagram of the L*b* plane of the CIE L*a*b* color spaceshowing the improved color achieved using the modified waveform shown inFIG. 20.

FIG. 22 is a schematic diagram of a DC balanced drive scheme asexplained in Example 9 below.

FIG. 23 is a graph showing a simple square wave waveform and resultantimpulse potentials, as described in Example 9 below.

FIG. 24 is a voltage against time graph for a waveform used for ablack-to-yellow transition in Example 9 below.

FIG. 25 is a graph showing the batch-to-batch variation of optimal drivevoltage obtained in experiments described in Example 9 below.

FIG. 26 shows voltage against time graphs for several picket fencewaveforms used in Example 9 below.

FIG. 27 is a graph showing the L* and b* values of the white statesobtained using the picket fence waveforms shown in FIG. 26.

DETAILED DESCRIPTION

As indicated above, the present invention provides various types ofcolor electrophoretic media and displays. However, all these types ofelectrophoretic media and displays rely upon a colored particle beingmoved in one direction along an electric field when the medium ordisplay is driven with a low addressing impulse and in the opposeddirection along the electric field when the medium or display is drivenwith a higher addressing impulse. This reversal of the direction ofmovement of the colored particle upon increasing the addressing impulsemay either be due to an actual reversal of the polarity of the charge onthe colored particle (as in the CSP media and displays of the presentinvention) or due to the colored particle forming part of an aggregatewith a second particle at low addressing impulse but becoming free fromthe aggregate at high addressing impulse (as in the SC and FC media anddisplays of the present invention).

It should be noted that, when using three subtractive primary coloredmaterials (i.e., cyan, magenta, and yellow) to render mixed colors,regardless of whether the three colored materials are present in thesame or different, stacked layers of an electrophoretic display, lightmust be selectively transmitted through at least two colored materialsbefore being reflected back to the viewer, either by a white reflector(if all three materials are light-transmissive), or by the thirdback-scattering material. The third colored material may belight-transmissive or reflective, as described in more detail below.Thus, when three such subtractive primary colored materials are used ina medium or display of the present invention, it is necessary for atleast two of the colored materials to be light-transmissive and notsubstantially back-scattering. Thus, for example, a magenta pigmentintended to absorb green light must transmit blue and red light tounderlying colored materials prior to the light being scattered back tothe viewer in order to render colors such as red or blue.

In regions where (for example) green light is not to be absorbed, it isnecessary that the green-absorbing, magenta colored material be removedfrom the optical path extending from the viewing surface of the displayto the location at which light is scattered back to the viewer. Thiscolored material removal may be achieved by concentrating the coloredmaterial in only a portion of the area of each pixel (thus, reducing itscovering power) when it is not intended to be seen, and spreading thecolored material over the whole pixel area when it is intended for themaximum amount of light to be absorbed. Hereinafter, spatiallyconcentrating a colored material so as to reduce its areal coveringpower is referred to as shuttering the material. In the media anddisplays of the present invention, unwanted pigment particles areremoved from the optical path not by shuttering, but by being concealedbehind light-scattering particles, as seen from the viewing surface ofthe display.

Displays of the present invention can, in this way, reproduce theappearance of high quality color printing. Such high quality printing istypically effected using at least three colorants in a subtractiveprimary color system, typically cyan/magenta/yellow (CMY) and optionallyblack. It is often not appreciated that a so-called three-color CMYprinting system is in reality a four-color system, the fourth colorbeing the white background provided by the substrate (paper or similar)surface to which colorants are applied, and which performs the functionof reflecting the light filtered by the subtractive colorants back tothe viewer. Since there is no comparable background color in anessentially opaque electrophoretic medium unless it is being used inshutter mode, a non-shutter-mode electrophoretic medium should becapable of modulating four colors (white and three primary colors, thethree primary colors typically being cyan, magenta and yellow, or red,green and blue). Optionally a black material may also be included, butit is possible to render black by a combination of cyan, magenta andyellow colors.

Before describing in detail preferred electrophoretic media and displaysof the present invention, some general guidance will be given regardingmaterials for use in such media and displays, and preferred processesfor their preparation.

The materials and processes used in preparing the media and displays ofthe present invention are generally similar to those used in similarprior art media and displays. As described for example incommonly-assigned U.S. Pat. No. 6,822,782, a typical electrophoreticmedium comprises a fluid, a plurality of electrophoretic particlesdisposed in the fluid and capable of moving through the fluid (i.e.,translating, and not simply rotating) upon application of an electricfield to the fluid. The fluid also typically contains at least onecharge control agent (CCA), a charging adjuvant, and a polymericrheology modifier. These various components will now be describedseparately.

A: Fluid

The fluid contains the charged electrophoretic particles, which movethrough the fluid under the influence of an electric field. A preferredsuspending fluid has a low dielectric constant (about 2), high volumeresistivity (about 10¹⁵ Ohm·cm), low viscosity (less than 5 mPas), lowtoxicity and environmental impact, low water solubility (less than 10parts per million (ppm), if traditional aqueous methods of encapsulationare to be used; note however that this requirement may be relaxed fornon-encapsulated or certain microcell displays), a high boiling point(greater than about 90° C.), and a low refractive index (less than 1.5).The last requirement arises from the use of scattering (typically white)pigments of high refractive index, whose scattering efficiency dependsupon a mismatch in refractive index between the particles and the fluid.

Organic solvents such as saturated linear or branched hydrocarbons,silicone oils, halogenated organic solvents, and low molecular weighthalogen-containing polymers are some useful fluids. The fluid maycomprise a single component or may be a blend of more than one componentin order to tune its chemical and physical properties. Reactants orsolvents for the microencapsulation process (if used), such as oilsoluble monomers, can also be contained in the fluid.

Useful organic fluids include, but are not limited to, saturated orunsaturated hydrocarbons (such as, but are not limited to, dodecane,tetradecane, the aliphatic hydrocarbons in the Isopar (Registered TradeMark) series (Exxon, Houston, Tex.), Norpar (Registered Trade Mark) (aseries of normal paraffinic liquids), Shell-Sol (Registered Trade Mark)(Shell, Houston, Tex.), and Sol-Trol (Registered Trade Mark) (Shell),naphtha, and other petroleum solvents; silicone oils (such as, but arenot limited to, octamethyl cyclosiloxane and higher molecular weightcyclic siloxanes, poly(methyl phenyl siloxane), hexamethyldisiloxane,and polydimethylsiloxane; vinyl ethers, such as cyclohexyl vinyl etherand Decave (Registered Trade Mark of International Flavors & Fragrances,Inc., New York, N.Y.); aromatic hydrocarbons, such as toluene; andhalogenated materials including, but not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene and carbon tetrachlorideand perfluoro- or partially-fluorinated hydrocarbons.

It is advantageous in some electrophoretic media of the presentinvention for the fluid to contain an optically absorbing dye. This dyemust be soluble or dispersible in the fluid, but will generally beinsoluble in the other components of the microcapsule. There is muchflexibility in the choice of dye material. The dye can be a purecompound, or blends of dyes may be used to achieve a particular color,including black. The dyes can be fluorescent, which would produce adisplay in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable by, for example, thermal, photochemical orchemical diffusion processes, forming a solid absorbing polymer insidethe bounding shell.

Many dyes can be used in electrophoretic media. Important dye propertiesinclude light fastness, solubility or dispersibility in the fluid,color, and cost. The dyes are generally chosen from the classes of azo,azomethine, fluoran, anthraquinone, and triphenylmethane dyes and may bechemically modified so as to increase their solubility in the fluid andreduce their adsorption to the particle surfaces.

B: Electrophoretic Particles

The electrophoretic particles used in the media and displays of thepresent invention are preferably white, black, yellow, magenta, cyan,red, green, or blue in color, although other (spot) colors may also beused. There is much flexibility in the choice of such particles. Forpurposes of this invention, an electrophoretic particle is any particlethat is insoluble in the fluid and charged or capable of acquiring acharge (i.e., has or is capable of acquiring electrophoretic mobility).In some cases, this mobility may be zero or close to zero (i.e., theparticles will not move). The particles may be, for example,non-derivatized pigments or dyed (laked) pigments, or any othercomponent that is charged or capable of acquiring a charge. Typicalconsiderations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light, i.e., the particles for use in the invention mayinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective or they may beelectroluminescent, such as zinc sulfide particles, or they may bephotoluminescent.

The electrophoretic particle may have any shape, i.e., spherical,plate-like or acicular. A scattering particle typically has highrefractive index, high scattering coefficient, and low absorptioncoefficient and may be composed of an inorganic material such as rutile(titania), anatase (titania), barium sulfate, zirconium oxide, kaolin,or zinc oxide. Other particles are absorptive, such as carbon black orcolored organic or inorganic pigments such as are used in paints andinks A reflective material can also be employed, such as a metallicparticle. Useful particle diameters may range from 10 nm up to about 10μm, although for light-scattering particles it is preferred that theparticle diameter not be smaller than about 200 nm.

Useful raw pigments for use in the electrophoretic particles include,but are not limited to, PbCrO₄, Cyan blue GT 55-3295 (American CyanamidCompany, Wayne, N.J.), Cibacron Black BG (Ciba Company, Inc., Newport,Del.), Cibacron Turquoise Blue G (Ciba), Cibalon Black BGL (Ciba),Orasol Black BRG (Ciba), Orasol Black RBL (Ciba), Acetamine Black, CBS(E. I. du Pont de Nemours and Company, Inc., Wilmington, Del.,hereinafter abbreviated du Pont), Crocein Scarlet N Ex (du Pont)(27290), Fiber Black VF (du Pont) (30235), Luxol Fast Black L (du Pont)(Solv. Black 17), Nirosine Base No. 424 (du Pont) (50415 B), Oil BlackBG (du Pont) (Solv. Black 16), Rotalin Black RM (du Pont), SevronBrilliant Red 3 B (du Pont); Basic Black DSC (Dye Specialties, Inc.),Hectolene Black (Dye Specialties, Inc.), Azosol Brilliant Blue B (GAF,Dyestuff and Chemical Division, Wayne, N.J.) (Solv. Blue 9), AzosolBrilliant Green BA (GAF) (Solv. Green 2), Azosol Fast Brilliant Red B(GAF), Azosol Fast Orange RA Conc. (GAF) (Solv. Orange 20), Azosol FastYellow GRA Conc. (GAF) (13900 A), Basic Black KMPA (GAF), Benzofix BlackCW-CF (GAF) (35435), Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black9), Celliton Fast Blue AF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA(GAF) (Basic Black 3), Diamine Black CAP Ex Conc (GAF) (30235), DiamondBlack EAN Hi Con. CF (GAF) (15710), Diamond Black PBBA Ex (GAF) (16505);Direct Deep Black EA Ex CF (GAF) (30235), Hansa Yellow G (GAF) (11680);Indanthrene Black BBK Powd. (GAF) (59850), Indocarbon CLGS Conc. CF(GAF) (53295), Katigen Deep Black NND Hi Conc. CF (GAF) (15711),Rapidogen Black 3 G (GAF) (Azoic Black 4); Sulphone Cyanine Black BA-CF(GAF) (26370), Zambezi Black VD Ex Conc. (GAF) (30015); Rubanox RedCP-1495 (The Sherwin-Williams Company, Cleveland, Ohio) (15630); Raven11 (Columbian Carbon Company, Atlanta, Ga.), (carbon black aggregateswith a particle size of about 25 μm), Statex B-12 (Columbian Carbon Co.)(a furnace black of 33 μm average particle size), Greens 223 and 425(The Shepherd Color Company, Cincinnati, Ohio 45246); Blacks 1, 1G and430 (Shepherd); Yellow 14 (Shepherd); Krolor Yellow KO-788-D (DominionColour Corporation, North York, Ontario; KROLOR is a Registered TradeMark); Red Synthetic 930 and 944 (Alabama Pigments Co., Green Pond, Ala.35074), Krolor Oranges KO-786-D and KO-906-D (Dominion ColourCorporation); Green GX (Bayer); Green 56 (Bayer); Light Blue ZR (Bayer);Fast Black 100 (Bayer); Bayferrox 130M (Bayer BAYFERROX is a RegisteredTrade Mark); Black 444 (Shepherd); Light Blue 100 (Bayer); Light Blue 46(Bayer); Yellow 6000 (First Color Co., Ltd., 1236-1, Jungwang-dong,Siheung-city, Kyonggi-do, Korea 429-450), Blues 214 and 385 (Shepherd);Violet 92 (Shepherd); and chrome green.

The electrophoretic particles may also include laked, or dyed, pigments.Laked pigments are particles that have a dye precipitated on them orwhich are stained. Lakes are metal salts of readily soluble anionicdyes. These are dyes of azo, triphenylmethane or anthraquinone structurecontaining one or more sulphonic or carboxylic acid groupings. They areusually precipitated by a calcium, barium or aluminum salt onto asubstrate. Typical examples are peacock blue lake (C1 Pigment Blue 24)and Persian orange (lake of C1 Acid Orange 7), Black M Toner (GAF) (amixture of carbon black and black dye precipitated on a lake).

It is preferred that pigments in the three subtractive primary colors(yellow, magenta and cyan) have high extinction coefficients andsufficiently small particle size as to be substantially non scatteringof incident light.

Particularly preferred raw pigment particles of the present inventionare the black spinels described in U.S. Pat. No. 8,270,064; titania,preferably with a silica, alumina or zirconia coating; red: Pigment Red112, Pigment Red 179, Pigment Red 188 and Pigment Red 254; green:Pigment Green 7; Blue: Pigment Violet 23; yellow: Pigment Yellow 74,Pigment Yellow 120, Pigment Yellow 138, Pigment Yellow 139, PigmentYellow 151, Pigment Yellow 155, and Pigment Yellow 180; magenta: PigmentViolet 19, Pigment Red 52:2 and Pigment Red 122; cyan: Pigment Blue15:2, Pigment Blue 15:3, Pigment Blue 15:4 and Pigment Blue 15:6.

Additional pigment properties which may be relevant are particle sizedistribution and light-fastness. Composite particle (i.e., polymericparticles that incorporate smaller pigment particles or dyes) may beused in the present invention. Pigments may be surface-functionalized asdescribed below or may be used without functionalization.

It has long been known that the physical properties and surfacecharacteristics of electrophoretic particles can be modified byadsorbing various materials on to the surfaces of the particles, orchemically bonding various materials to these surfaces; see U.S. Pat.No. 6,822,782, especially column 4, line 27 to column 5, line 32. Thissame U.S. patent demonstrates that there is an optimum amount of polymerwhich should be deposited (too large a proportion of polymer in themodified particle causes an undesirable reduction in the electrophoreticmobility of the particle) and that the structure of the polymer used toform the coating on the particle is important.

C: Charge Control Agents

The electrophoretic media of the present invention will typicallycontain a charge control agent (CCA), and may contain a charge director.These electrophoretic media components typically comprise low molecularweight surfactants, polymeric agents, or blends of one or morecomponents and serve to stabilize or otherwise modify the sign and/ormagnitude of the charge on the electrophoretic particles. The CCA istypically a molecule comprising ionic or other polar groupings,hereinafter referred to as head groups. At least one of the positive ornegative ionic head groups is preferably attached to a non-polar chain(typically a hydrocarbon chain) that is hereinafter referred to as atail group. It is thought that the CCA forms reverse micelles in theinternal phase and that it is a small population of charged reversemicelles that leads to electrical conductivity in the very non-polarfluids typically used as electrophoretic fluids.

Reverse micelles comprise a highly polar core (that typically containswater) that may vary in size from 1 nm to tens of nanometers (and mayhave spherical, cylindrical, or other geometry) surrounded by thenon-polar tail groups of the CCA molecule. Reverse micelles have beenextensively studied, especially in ternary mixtures such asoil/water/surfactant mixtures. An example is the iso-octane/water/AOTmixture described, for example, in Fayer et al., J. Chem. Phys., 131,14704 (2009). In electrophoretic media, three phases may typically bedistinguished: a solid particle having a surface, a highly polar phasethat is distributed in the form of extremely small droplets (reversemicelles), and a continuous phase that comprises the fluid. Both thecharged particles and the charged reverse micelles may move through thefluid upon application of an electric field, and thus there are twoparallel pathways for electrical conduction through the fluid (whichtypically has a vanishingly small electrical conductivity itself).

The polar core of the CCA is thought to affect the charge on surfaces byadsorption onto the surfaces. In an electrophoretic display, suchadsorption may be onto the surfaces of the electrophoretic particles orthe interior walls of a microcapsule (or other solid phase, such as thewalls of a microcell) to form structures similar to reverse micelles,these structures hereinafter being referred to as hemi-micelles. Whenone ion of an ion pair is attached more strongly to the surface than theother (for example, by covalent bonding), ion exchange betweenhemi-micelles and unbound reverse micelles can lead to charge separationin which the more strongly bound ion remains associated with theparticle and the less strongly bound ion becomes incorporated into thecore of a free reverse micelle.

It is also possible that the ionic materials forming the head group ofthe CCA may induce ion-pair formation at the particle (or other)surface. Thus the CCA may perform two basic functions: charge-generationat the surface and charge-separation from the surface. Thecharge-generation may result from an acid-base or an ion-exchangereaction between some moiety present in the CCA molecule or otherwiseincorporated into the reverse micelle core or fluid, and the particlesurface. Thus, useful CCA materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Non-limiting classes of charge control agents which are useful in themedia of the present invention include organic sulfates or sulfonates,metal soaps, block or comb copolymers, organic amides, organiczwitterions, and organic phosphates and phosphonates. Useful organicsulfates and sulfonates include, but are not limited to, sodiumbis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate,calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalene sulfonate,dodecylbenzenesulfonic acid sodium salt, and ammonium lauryl sulfate.Useful metal soaps include, but are not limited to, basic or neutralbarium petronate, calcium petronate, cobalt, calcium, copper, manganese,magnesium, nickel, zinc, aluminum and iron salts of carboxylic acidssuch as naphthenic, octanoic, oleic, palmitic, stearic, and myristicacids and the like. Useful block or comb copolymers include, but are notlimited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides/amines include, but are not limited to, polyisobutylenesuccinimides such as OLOA 371 or 1200 (available from Chevron OroniteCompany LLC, Houston, Tex.), or Solsperse 17000 (available fromLubrizol, Wickliffe, Ohio: Solsperse is a Registered Trade Mark), andN-vinylpyrrolidone polymers. Useful organic zwitterions include, but arenot limited to, lecithin. Useful organic phosphates and phosphonatesinclude, but are not limited to, the sodium salts of phosphated mono-and di-glycerides with saturated and unsaturated acid substituents.Useful tail groups for CCA include polymers of olefins such aspoly(isobutylene) of molecular weight in the range of 200-10,000. Thehead groups may be sulfonic, phosphoric or carboxylic acids or amides,or alternatively amino groups such as primary, secondary, tertiary orquaternary ammonium groups.

Charge adjuvants used in the media of the present invention may bias thecharge on electrophoretic particle surfaces, as described in more detailbelow. Such charge adjuvants may be Bronsted or Lewis acids or bases.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule or other walls or surfaces.For the typical high resistivity liquids used as fluids inelectrophoretic displays, non-aqueous surfactants may be used. Theseinclude, but are not limited to, glycol ethers, acetylenic glycols,alkanolamides, sorbitol derivatives, alkyl amines, quaternary amines,imidazolines, dialkyl oxides, and sulfosuccinates.

D: Polymeric Additives

As described in U.S. Pat. No. 7,170,670, the bistability ofelectrophoretic media can be improved by including in the fluid apolymer having a number average molecular weight in excess of about20,000, this polymer being essentially non-absorbing on theelectrophoretic particles; poly(isobutylene) is a preferred polymer forthis purpose.

Also, as described in for example, U.S. Pat. No. 6,693,620, a particlewith immobilized charge on its surface sets up an electrical doublelayer of opposite charge in a surrounding fluid. Ionic head groups ofthe CCA may be ion-paired with charged groups on the electrophoreticparticle surface, forming a layer of immobilized or partiallyimmobilized charged species. Outside this layer is a diffuse layercomprising charged (reverse) micelles comprising CCA molecules in thefluid. In conventional DC electrophoresis an applied electric fieldexerts a force on the fixed surface charges and an opposite force on themobile counter-charges, such that slippage occurs within the diffuselayer and the particle moves relative to the fluid. The electricpotential at the slip plane is known as the zeta potential.

The electrophoretic motion of charged particles in a fluid is covered inmost textbooks on colloid science. See, for example, Hiemenz, P. C. andRajagopalan, R., Principles of Colloid and Surface Chemistry, 3rd ed.,Marcel Dekker, N Y, 1997. In systems of interest for electrophoreticdisplays, the dielectric constant is usually low (in the range of 2-10),and the number of ions small. In this regime the following equationholds:

$\begin{matrix}{\zeta = \frac{q}{4\;{\prod{ɛ_{0}ɛ_{r}a}}}} & (1)\end{matrix}$where ζ is the zeta potential; q is the net charge on the particle; ε₀is the vacuum permittivity constant; ε_(r) is the dielectric constant;and a is the particle radius. Note that a particle having a zetapotential of ˜50 mV and a radius of ˜150 nm therefore has a net chargeof only about 10 electronic charge units in a medium of dielectricconstant 2.

This concludes the general discussion of the components ofelectrophoretic media and displays. Preferred electrophoretic media anddisplays of the present invention will now be described with referenceto the accompanying drawings.

FIG. 1 of the accompanying drawings is a schematic cross-section throughan electrophoretic display (generally designated 100) of the presentinvention comprising an encapsulated electrophoretic medium; such adisplay, and methods for its manufacture are described in U.S. Pat. No.6,982,178. The display 100 comprises a light-transmissive substrate 102,typically a transparent plastic film, such as a sheet of poly(ethyleneterephthalate) (PET) about 25 to 200 μm in thickness. Although not shownin FIG. 1, the substrate 102 (the upper surface of which, as illustratedin FIG. 1, forms the viewing surface of the display) may comprise one ormore additional layers, for example a protective layer to absorbultra-violet radiation, barrier layers to prevent ingress of oxygen ormoisture into the display, and anti-reflection coatings to improve theoptical properties of the display.

The substrate 102 carries a thin, light-transmissive,electrically-conductive layer 104 that acts as the front electrode ofthe display. Layer 104 may comprise a continuous coating ofelectrically-conductive material with minimal intrinsic absorption ofelectromagnetic radiation in the visible spectral range such as indiumtin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT:PSS), graphene and the like, or may be a discontinuous layer of amaterial such as silver (in the form of, for example, nanowires orprinted grids) or carbon (for example in nanotube form) that absorb orreflect visible light but are present at such a surface coverage thatthe layer as a whole is effectively transparent.

A layer (generically designated 108) of an electrophoretic medium is inelectrical contact with the conductive layer 104 through an optionalpolymeric layer or layers 106, as described in more detail below. Theelectrophoretic medium 108 is shown as an encapsulated electrophoreticmedium comprising a plurality of microcapsules. The microcapsules may beretained within a polymeric binder. Upon application of an electricalfield across the layer 108, negatively-charged particles therein movetowards the positive electrode and positively-charged particles movetowards the negative electrode, so that the layer 108 appears, to anobserver viewing the display through the substrate 102, to change color.

Although the display 100 is illustrated as having an encapsulatedelectrophoretic layer 108, this is not an essential feature of thepresent invention. Layer 108 may be encapsulated or comprise sealed orunsealed micro-cells or micro-cups, or may be non-encapsulated. When thelayer is non-encapsulated, the electrophoretic internal phase (theelectrophoretic particles and fluid) may be located between two planarelectrodes, at least one of which is light-transmissive. The spacingbetween the electrodes may be controlled by the use of spacers, whichmay have the form of ribs or beads. Alternatively, the spacing may becontrolled by the use of microcapsules containing the internal phase;the internal phase may be located within and outside the capsules. It isnot necessary that the internal phase inside and outside themicrocapsules be identical, although in certain circumstances this maybe preferred. For example, if capsules containing the same internalphase as that outside the capsules are used as spacers it may be thatthe presence of the spacers is less easily discernible by a viewer ofthe display (since the internal and external internal phases wouldswitch to at least substantially the same color).

As described in U.S. Pat. Nos. 6,982,178 and 7,012,735, the display 100further comprises a layer 110 of lamination adhesive covering theelectrophoretic layer 108. The lamination adhesive makes possible theconstruction of an electro-optic display by combining two subassemblies,namely a backplane 118 that comprises an array of pixel electrodes 112and an appropriate arrangement of conductors to connect the pixelelectrodes to drive circuitry, and a front plane 116 that comprises thesubstrate 102 bearing the transparent electrode 104, the electrophoreticlayer 108, the lamination adhesive 110 and optional additionalcomponents such as polymeric layer or layers 106. To form the finaldisplay, the front plane 116 is laminated to the backplane 118 by meansof lamination adhesive 110. The lamination adhesive may be curedthermally or by actinic radiation (for example, by UV curing) or may beuncured.

Since the lamination adhesive 110 is in the electrical path from thebackplane electrodes 112 to the front electrode 104, its electricalproperties must be carefully tailored. As described in U.S. Pat. No.7,012,735 the lamination adhesive may comprise, in addition to apolymeric material, an ionic dopant that may be an additive selectedfrom a salt, a polyelectrolyte, a polymer electrolyte, a solidelectrolyte, a conductive metal powder, a ferrofluid, a non-reactivesolvent, a conductive organic compound, and combinations thereof. Thevolume resistivities of encapsulated electrophoretic media of thepresent invention are typically around 10¹⁰ Ohm·cm, and theresistivities of other electro-optic media are usually of the same orderof magnitude. Accordingly, the volume resistivity of the laminationadhesive is normally around 10⁸ to 10¹² Ohm·cm at the operatingtemperature of the display, which is typically around 20° C.

Polymeric layer 106 may be a lamination adhesive layer with similarproperties to those of lamination adhesive layer 110 (see for exampleU.S. Pat. No. 7,839,564), except that, since polymeric layer 106 isadjacent to the non-pixelated, light-transmissive common electrode 104,its electrical conductivity may be higher than that of laminationadhesive layer 110, which is adjacent to the pixelated back planeelectrodes 112 and cannot be so conductive as to lead to significantcurrents flowing from one backplane electrode to its neighbors when theyare held at different potentials during switching of the display. Whenpolymeric layer 106 is a lamination adhesive it may be used to affixelectrophoretic layer 108 to front electrode 104 during manufacture ofthe front plane as described in detail in the aforementioned U.S. Pat.No. 6,982,178.

FIG. 2 illustrates schematically the flux of charged materials that mayoccur within an electrophoretic medium of the present inventioncontained in a cell (generally designated 200) in response to anelectric field applied by means of electrodes 202 and 204. Mobilecharged species in the electrophoretic fluid are shown generically ascharged particles P+ and P− and charged (reverse) micelles RM+ and RM−.Upon the application of an electric field, the charged species move, andas they do so, screen the field in the interior of the cell. If theelectrodes are blocked (i.e., the electrodes do not allow the passage ofan electrochemical current) the charged species pile up at the electrodeinterfaces until the field at the mid-point between the electrodes dropsto zero. This polarization process can be thought of as a charging ofinterfacial capacitors by conduction through the electrophoreticinternal phase (although, as will be appreciated by those of skill inthe art, the situation is more complex than this basic picture wouldsuggest, since dissociation of neutral reverse micelles to chargedmicelles can lead to charge generation within the internal phase).

The movement of charged species in an arrangement such as thatillustrated in FIG. 2 with blocked electrodes can be modeled using thePoisson-Nernst-Planck system of partial differential equations. Usingthe Butler-Volmer-Frumkin equation, the effect of electrochemicalreactions at the electrodes can be incorporated into such a model. FIGS.3A and 3B show the results of such modeling, and illustrate theelectrical potential within an electrophoretic internal phase separatingtwo electrodes as a function of the distance from a first electrode.FIG. 3A shows the evolution of this potential with time in the casewhere the electrodes are blocked; a large potential drop develops at theelectrode interfaces, while the potential gradient at the center of thecell becomes zero. Therefore, after the polarization of the cell, thenet flow of electrophoretic particles (and reverse micelles) ceases asdrift and thermal diffusion balance each other. Note that the internalphase would experience an electric field equal and opposite to theinitially applied field if, after complete polarization, the electrodeswere both connected to ground (or brought to a common potential). Thiswould result in erasure of any image on the medium (the so-calledkick-back problem).

FIG. 3B illustrates the case in which electrochemical reactions (chargeinjection) can occur at the electrode interfaces. In this case, after aninitial polarization, the electric field in the vicinity of theelectrodes becomes high enough that electrons are transferred in bothdirection between molecules in the internal phase and the electrodes.Materials are oxidized at the anode and reduced at the cathode. Providedthat there is a sufficient supply of redox-active materials adjacent theelectrodes, a steady-state current flows in the cell 200 (and thepotential gradient at the center of the cell is non-zero).

FIG. 4 is a schematic cross-section similar to that of FIG. 2 butillustrates the case (just discussed with reference to FIG. 3B) whereelectrochemical reactions occur at the electrode interfaces. In FIG. 4there is shown a unipolar electrochemical current; i.e., ions aregenerated at one electrode and consumed at the other. Thus, at the anode204, species A loses an electron with the production of a proton, whichtravels through the internal phase as shown by arrow 206 to the cathode202, where it is reduced to a neutral hydrogen species, shown in FIG. 4as H, which may be hydrogen gas.

It is believed (although the invention is in no way limited by thisbelief) that one of the electrochemical reactions occurring inelectrophoretic media of the present invention is water electrolysis. Inpure water an oxidation of water takes place at the anode, with theproduction of protons, thus:Anode (oxidation): H₂O→½O₂+2H⁺+2e ⁻  (2)At the cathode, protons are reduced with the production of hydrogen (orother hydrogen radical products):Cathode (reduction): 2H⁺+2e ⁻→H₂  (3)The net effect of these electrochemical reactions is to transfer protonsfrom the anode to the cathode, with the consumption of water, asillustrated by the bold arrow 206 in FIG. 4. Note that the unipolar(one-directional) transfer of protons is in the opposite direction tothe travel of the negatively-charged reverse micelles and pigments.

As mentioned above, the present invention provides an electrophoreticmedium comprising a fluid and at least a first species of particlesdisposed in the fluid, the first species of particles being such thatwhen a first electric field is applied to the medium for a first period,thereby applying a first addressing impulse to the medium, the firstspecies of particles move in one direction relative to the electricfield, but when a second electric field, having the same polarity as thefirst electric field, is applied to the medium for a second period,thereby applying a second addressing impulse larger than the firstaddressing impulse to the medium, the first species of particles move inthe opposed direction relative to the electric field. For the purpose ofproviding a better understanding of the present invention, the followinghypothesis as to how the pigment particles might move in a firstdirection with a first addressing impulse (i.e., behaving as though theparticles bore a negative charge) and in a second direction with asecond, higher addressing impulse (i.e., behaving as though theparticles bore a positive charge) is provided, but the invention is inno way limited by this hypothesis.

FIG. 5A shows a negatively-charged particle 500, having a net charge offour electronic charge units, that would, in the absence of anelectrochemical current, move in the direction of the dashed arrow inFIG. 5A. Adsorbed on the particle 500 is a layer of hemi-micellarmaterial 502 of high dielectric constant. (FIGS. 5A and 5B are not drawnto scale, and that the shapes of the particle and adsorbed layer areshown in idealized form as concentric ellipses. In fact the adsorbedlayer is likely to be polarized, and may in fact not be a continuouslayer nor of constant thickness.

The material of layer 502 is likely to have a similar composition to thecore material in reverse micelles and contain water. Four surface-bound(or surface-adsorbed) negative charges 504 are illustrated, incorporatedinto the layer 502 of high dielectric constant. The counter-ions tothese four charges form a non-attached, micellar diffuse layer (notshown) that can move in a direction opposite to that of the particle inan electric field, because these charges lie beyond the hydrodynamicslip envelope surrounding the particle. This is the normal condition forelectrophoretic motion in a suspending liquid of low dielectric constantcontaining reverse micellar charged species.

As shown in FIG. 5B, the situation is thought to be different when acurrent, carried mostly by reverse micelles, flows through the internalphase. Such a current may result from electrochemical reactions at theelectrodes or from displacement of charges from layers adjoining theinternal phase. It is thought that the current is predominantly carriedby charge-carriers of one sign; this may arise from particularelectrochemical reactions (as mentioned above) or from differingmobilities of charge carriers of opposite polarities. In FIG. 5B it isassumed that the charge carriers are predominantly positively-charged,as would be the case for electrolysis of water at neutral or acidic pH.

As previously mentioned, conduction within the highly non-polar fluid islikely to be mediated by charged reverse micelles or chargedelectrophoretic particles. Therefore, any electrochemically-generatedprotons (or other ions) are likely to be transported through the fluidin micelle cores or adsorbed on electrophoretic particles. In FIG. 5B apositively-charged reverse micelle 506 is shown approaching the particle500, and traveling in the opposite direction from that in which theparticle 500 would have traveled during polarization of the internalphase. A reverse micelle approaching a much larger particle may travelpast the particle without interaction, or may be incorporated into theelectric double layer around the oppositely charged particle. Theelectric double layer includes both the diffuse layer of charge withenhanced counter-ion concentration and the hemi-micellarsurface-adsorbed coating on the particle; in the latter case, thereverse micelle charge would become associated with the particle withinthe slip envelope that, as noted above, defines the zeta potential ofthe particle. Therefore, while an electrochemical current ofpositively-charged ions is flowing, it is hypothesized thatnegatively-charged particles may become biased towards a more positivecharge as a result of a type of ionic exchange at the particle surfacedriven by the transport of ions (such as ion 508 in FIG. 5B) through thefluid in micelle cores. It is further hypothesized that a reversemicelle might bud off from the particle as shown by reverse micelle 510in FIG. 5B. The net charge on the particle would therefore be a functionof the magnitude of the electrochemical current and the residence timeof a positive charge close to the particle surface. This residence timeis likely to be affected by the particle size, its surface chemistry,and the chemical nature of the micelles in the fluid and thehemi-micelles adsorbed onto the particle surface.

FIG. 6 is a micrograph illustrating the result of one experimentintended to confirm the hypothesis set out above by tracing particlemotions. FIG. 6 shows interdigitated electrodes disposed on a glasssurface, separated by a 25 μm gap. A cell is constructed with theinterdigitated electrodes as one boundary and a flat piece of glass asthe second boundary. Between the two boundaries is placed an internalphase comprising a fluid (Isopar V), a CCA (Solsperse 17000) and aplurality of white (light-scattering) polymer-coated titania particlesbearing a negative charge and having a zeta potential of approximately−40 mV; the polymer-coated titania was prepared substantially asdescribed in Example 28 of U.S. Pat. No. 6,822,782. When one electrodeis grounded and its neighbor is at +40V, the white particles collect atthe more positive electrode. As the voltage is increased (and thecurrent flow correspondingly increased) the white particles move towardsthe less positive electrode until, at +160V, the majority of the whiteparticles collect in the vicinity of the less positive electrode.Because of the thickness of the experimental cell used, voltages used inthis experiment are much higher than would be required in a commercialelectrophoretic display, which uses a much thinner layer ofelectrophoretic medium.

FIG. 7, which is schematic cross-section, similar to that of FIG. 4,through an electrophoretic display (generally designated 700) embodyingthe above principles. An electrophoretic medium layer 706 is sandwichedbetween polymeric lamination adhesive layers 708 and 710; only a singlelamination adhesive layer need be used if an (encapsulated) internalphase fluid is coated directly onto one of electrodes 704 and 712. Theelectrochemical current flowing through the device is illustrated by thebold arrows. As noted above, the lamination adhesives may be doped withmobile ionic species, and it is not necessary that the (unipolar)current flowing between the anode 712 and the cathode 704 be carried bya common ion. Thus, cationic species B+ is shown as crossing theboundary between the lamination adhesive layer 710 and theelectrophoretic layer 706, where B+ may be a proton or another positiveion. Likewise, ion C+ is shown crossing the boundary between theelectrophoretic layer 706 and the lamination adhesive layer 708. Again,C+ could be a proton or another cation. At the cathode 704 itself, aproton is shown being reduced but this is not an essential feature ofthe present invention.

As noted above, in the present invention that the current flowing in thedevice is not necessarily electrochemical in origin; a displacementcurrent comprising flow of ions B+ and C+ can suffice to induce thereversed direction motion of the negatively charged particles, providedthat such a displacement current is unipolar. The above argument, inwhich it is assumed that the electrochemical or displacement current isunipolar in positively-charged ions, and can therefore lead todirection-reversal of negatively-charged particles, applies to aunipolar electrochemical current in negatively-charged ions, in whichcase it would be positive-charged particles which would reversedirection. In practice, however, it has been found easier to engineerdirection-reversal of negatively-charged particles.

Furthermore, the unipolar nature of the current is not a requirement ofthe present invention, although it is easier to understand the observedphenomena (for example, the behavior documented in FIG. 6) if a unipolarcurrent is assumed.

Various embodiments of electrophoretic media and displays of the presentinvention, and their use to form colored images, will now be describedin more detail. In these embodiments the following general switchingmechanisms are utilized:

-   -   (A) Conventional electrophoretic motion, in which particles with        associated charge (either surface-bound or adsorbed) move in an        electric field;    -   (B) Conventional racing particles, wherein particles of higher        zeta potential move faster than particles of lower zeta        potential (as described, for example, in U.S. Pat. No. 8,441,714        and earlier patents cited therein)    -   (C) Coulombic aggregation between particles of opposite sign,        such that the aggregate moves in an electric field according to        its net charge in the absence of an electrochemical (or        displacement) current, but wherein the aggregate is separated by        modulation of charge on at least one of the particles by the        electrochemical (or displacement) current;    -   (D) Reversal of the direction of motion of at least one species        of particles as a result of electrochemical (or displacement)        current.

The waveforms used to drive displays of the present invention maymodulate the electrical impulse provided to the display using any one ormore of at least four different methods:

-   -   (i) Pulse width modulation, in which the duration of a pulse of        a particular voltage is changed;    -   (ii) Duty cycle modulation, in which a sequence of pulses is        provided whose duty cycle is changed according to the impulse        desired;    -   (iii) Voltage modulation, in which the voltage supplied is        changed according to the impulse required; and    -   (iv) A DC voltage offset applied to an AC waveform (which itself        has net zero impulse)

Which of these methods is used depends upon the intended application andthe exact form of display used. As noted above, herein the term impulseis used to denote the integral of the applied voltage with respect totime during the period in which a medium or display is addressed. Alsoas noted above, a certain electrochemical or displacement current isrequired for the change in direction of a (typically negatively-charged)species of particle or the disaggregation of Coulombic aggregates, andthus when a high impulse is to applied to a medium of display, theaddressing voltage must be sufficient to provide such a current. Lowerimpulses may be provided by lower addressing voltages, or by reductionin the addressing time at the same higher voltage. As noted above, thereis a polarization phase during which electrochemical currents are not attheir maximum value, and during this polarization phase the particlesmove according to their native charge (i.e., the charge they bear beforeany addressing voltage is applied to the medium or display. Thus,low-impulse addressing at high voltage is ideally for a duration such asto polarize the electrophoretic medium but not lead to high steady-statecurrent flow.

FIGS. 8A and 8B are schematic cross-sections showing various possiblestates of single microcapsule 800 (a sealed or unsealed microcell, orother similar enclosure may alternatively be used), containing a fluid806 dyed with a yellow dye (uncharged yellow particles may besubstituted for the yellow dye. Disposed in the fluid 806 arepositively-charged light-transmissive magenta particles 802 andnegatively-charged white particles 804. On the upper side ofmicrocapsule 800, as illustrated in FIGS. 8A and 8B, is a substantiallytransparent front electrode 810, the upper surface of which (asillustrated) forms the viewing surface of the display, while on theopposed side of the microcapsule 800 is a rear or pixel electrode 812.In FIGS. 8A and 8B, and in similar later Figures, it will be assumedthat the front electrode 810 remains at ground potential (although thisis not an essential feature of the present invention, and variation ofthe potential of this electrode may be desirable in some instances, forexample to provide higher electric fields), and that the electric fieldacross microcapsule 800 is controlled by changing the voltage of therear electrode 812.

FIG. 8A illustrates the two possible states of the microcapsule 800 whendriven with a low impulse. Under such a low impulse, the particles 802and 804 undergo conventional electrophoretic motion. As shown on theleft hand side of FIG. 8A, when the rear electrode 812 is at a positivevoltage, the white particles 804 move towards the rear electrode 812,while the magenta particles 802 lie adjacent the front electrode 810, sothat the microcapsule 800 displays a red color caused by the combinationof the magenta particles and the yellow dye viewed against the whitebackground provided by the white particles. As shown on the right handside of FIG. 8A, when the rear electrode 812 is at a negative voltage,the white particles 804 move adjacent the front electrode 810, and themicrocapsule 800 displays a white color (both the yellow fluid 806 andthe magenta particles 802 are masked by the white particles 804).

FIG. 8B illustrates the two possible states of the microcapsule 800 whendriven with a high impulse. Under such a high impulse, the magentaparticles 802 continue to undergo conventional electrophoretic motion.However, the white particles 804 undergo charge reversal and behave asif they were positively charged. Accordingly, as shown on the left handside of FIG. 8B, when the rear electrode 812 is at a positive voltage,the magenta particles 802 lie adjacent the front electrode 810, but thewhite particles 804 also move towards the front electrode and aredisposed deposited directly below the magenta particles, so that themicrocapsule 800 displays a magenta color (light passing from theviewing surface through the magenta particles 802 is reflected from thewhite particles 804 back through the magenta particles 802 and outthrough the viewing surface; the white particles 804 mask the yellowfluid 806). As shown on the right hand side of FIG. 8B, when the rearelectrode 812 is at a negative voltage, the white particles 804 movetowards the rear electrode 812 and are disposed above the magentaparticles 802, so the microcapsule displays a yellow color (lightpassing from the viewing surface is filtered by the yellow fluid 806 andreflected from the white particles 804 back through the yellow fluid 806and out through the viewing surface; the white particles 804 mask themagenta particles 802). Thus, at low addressing impulses thecomplementary color pair white/red is produced, while at high addressingimpulses the color pair yellow/magenta is produced.

Obviously, other combinations of colored particles and dyes can besubstituted for the white and magenta particles, and yellow dye, used inFIGS. 8A and 8B. Especially preferred embodiments of the presentinvention are those in which one dye or particle has one of the additiveprimary colors, and another is of the complementary subtractive primarycolor. Thus, for example, the dye might be cyan and the two particleswhite and red. The four states afforded by this combination are whiteand black (at low impulse driving) and red and cyan (at high impulsedriving). Similarly, green/magenta and blue/yellow combinations of dyeand particle may be used, together with a white particle.

FIGS. 9, 10A and 10B illustrate another embodiment of the presentinvention intended to provide black, white, and a single spot color. Itis desirable in such displays to be able to provide intermediate graylevels between white and black, between white and the spot color, andbetween black and the spot color. FIGS. 10A and 10B illustrate a displayin which the spot color is yellow.

FIG. 9 is a graph showing the CIE L* (lightness) and CIE C* (chroma)values obtainable from a display of this type. The display can bewritten to black, white, or the spot color (assumed yellow), and canattain intermediate states of gray (arrow 904), black/yellow (arrow 906)and white/yellow (arrow 902). For applications such as electronic bookreaders providing having a spot (highlight) color in addition to blackand white, it is important for text and image rendering that theseintermediate states be available.

FIGS. 10A and 10B illustrate schematically the various possible statesof a display of the present invention. This display comprises amicrocapsule 1000 having a grounded front electrode 1010 (the uppersurface of which, as illustrated, provides the viewing surface of thedisplay) and a rear electrode 1012. All these integers are essentiallyidentical to the corresponding integers in FIGS. 8A and 8B. However, thefluid within the microcapsule 1000 is not dyed but has disposed thereinthree species of particles, namely positively-charged black particles1008, positively-charged colored particles 1002 (illustrated as yellow)and negatively-charged white particles 1004. The yellow particles 1002are light-transmissive and preferably substantially non-scattering. Thecharges are shown on the particles 1002, 1004 and 1008 are indicated as+2, −3 and +8 respectively, but these are for the purposes ofillustration only and do not limit the scope of the present invention.

Black particles 1008 bear a polymeric coating (shown in FIGS. 10A and10B as a bold outline). Yellow particles 1002 bear no polymeric coating,or a polymeric coating of lower coverage per unit area of the particlesthan that borne by the black particles 1008, and white particles 1004also bear no polymeric coating, or a polymeric coating of lower coverageper unit area of the particles than that borne by the black particles1008. The polymeric coating on the black particles 1008 ensures that aspacing is maintained between the black particles 1008 and the whiteparticles 1004, such that any Coulombic aggregate formed between theparticles 1004 and 1008 is sufficiently weak to be separated by a lowaddressing impulse. On the other hand, the absence or minimal amount ofpolymer on the yellow particles 1002 and the white particles 1004enables much stronger aggregation between these two types of particlessuch that the aggregate is not separated by a low addressing impulse butcan be separated by a high addressing impulse, as described in moredetail below.

More generally, the Hamaker constant (which is a measure of the strengthof the Van der Waals interaction between two particles, the pairpotential being proportional to the Hamaker constant and inverselyproportional to the sixth power of the distance between the twoparticles) and/or the interparticle spacing need(s) to be adjusted byjudicious choice of the polymeric coating such that the particle pairinteractions, both Coulombic and attractive non-Coulombic, are lessbetween the white particles and the black particles than between thewhite particles and the yellow particles.

The effect of these inter-particles interactions is that, in the absenceof an addressing impulse, or in the presence of a low addressingimpulse, a Coulombic aggregate is formed between the yellow particles1002 and the white particles 1004, and the two travel together as aweakly negatively-charged unit. Thus, in the absence of anelectrochemical or displacement ionic current, the internal phase withinthe microcapsule 1000 fluid behaves as if it contained positive blackand negative yellow particles. Under high-impulse driving conditions, onthe other hand, the white particles 1004 and (possibly) the yellowparticles 1002 are moved to a more positively charged state by the fluxof electrochemically- or displacement-generated positive ions, asdescribed above, and the strength of the Coulombic aggregate formedbetween the white and yellow particles is weakened. The electric fieldis now sufficient to separate the two types of particles, so that theyellow particles 1002 now move with the black particles 1008 towards themore negative electrode, while the white particles 1004, though moreweakly negatively charged than they would be in the absence of anelectric current through the internal phase, still migrate towards themore positive electrode.

The effect of these changes is shown in FIGS. 10A and 10B. The left handside of FIG. 10A shows the microcapsule 1000 under low impulse drivingconditions and with the rear electrode 1012 positive relative to thefront electrode 1010. The black particles 1008 migrate to the frontelectrode 1010, while the white particle/yellow particle aggregates(which remain intact under the low impulse driving) migrate to the rearelectrode 1012. The black particles mask the white and yellow particles,so that the microcapsule shows a black color. The right hand side ofFIG. 10A shows the microcapsule 1000 under low impulse drivingconditions but with the rear electrode 1012 negative relative to thefront electrode 1010. The black particles 1008 migrate to the rearelectrode 1010, while the white particle/yellow particle aggregatesmigrate to the front electrode 1010. The microcapsule 1000 thus displaysa yellow color.

On the other hand, the left hand side of FIG. 10B shows the microcapsule1000 under high impulse driving conditions and with the rear electrode1012 positive relative to the front electrode 1010. The high impulsedriving conditions disrupt the white particle/yellow particleaggregates, so that the white, yellow and black particles all moveindependently of one another. Accordingly, the black and yellowparticles move adjacent the front electrode 1010, while the whiteparticles move adjacent the rear electrode 1012, and the microcapsuledisplays a black color; the light-transmissive yellow particles do notaffect the black color of this state since the light-absorbing blackparticles absorb all light incident on the viewing surface and alsoserve to mask the white particles. The right hand side of FIG. 10B showsthe microcapsule 1000 under high impulse driving conditions but with therear electrode 1012 negative relative to the front electrode 1010. Theblack and yellow particles move adjacent the rear electrode 1012, whilethe white particles move adjacent the front electrode 1010, and mask theblack and yellow particles. Thus, the microcapsule displays a whitecolor.

Thus, under low impulse driving conditions, the microcapsule 1000 can beswitched between black and yellow states, while under high impulsedriving conditions, the microcapsule can be switched between black andwhite states.

The driving sequence to display the spot color (yellow in FIG. 10A) andgray levels is as follows. Using a high impulse driving condition withthe backplane positive (the left hand side of FIG. 10B), the display isdriven to black. Then, using a low impulse driving condition with thebackplane negative (the right hand side of FIG. 10A), the display isdriven to yellow (following arrow 906 in FIG. 9. From the yellow state,high impulse driving with the backplane negative (the right hand side ofFIG. 10B) produces a white state (following arrow 902 in FIG. 9).Finally, driving from white with a low impulse driving condition and thebackplane positive (the left hand side of FIG. 10A) provides gray levelson the way to black (following arrow 904 in FIG. 9).

FIGS. 11A-11D of the accompanying drawings illustrate various states ofa display which uses the principles of the displays shown in FIGS. 8A,8B, 10A and 10B, and three different levels of driving impulse toprovide a display in which each microcapsule is capable of displayingblack and white and both the additive and subtractive primary colors(red, green, blue, cyan, magenta and yellow). In FIGS. 11A-11D, particlecharges are shown for purposes of illustration only, and in no way limitthe scope of the present invention.

FIGS. 11A-11D show a microcapsule 1100 having a grounded front electrode1110 (the upper surface of which, as illustrated, provides the viewingsurface of the display) and a rear electrode 1112. All these integersare essentially identical to the corresponding integers in FIGS. 8A, 8B,10A and 10B. The microcapsule 1100 contains a fluid 1108 dyed with acyan dye. The fluid 1108 but has disposed therein three species ofparticles, namely positively-charged light-transmissive magentaparticles 1102, positively-charged light-transmissive yellow particles1104 and negatively-charged light-scattering white particles 1106. Themagenta particles 1102 bear a polymeric coating, whereas the white andyellow particles 1104 and 1106 bear no or only a thin polymeric coating.Accordingly, under low impulse driving conditions, as shown in FIG. 11A,the microcapsule 1100 acts in a manner exactly analogous to themicrocapsule 1000 shown in FIGS. 10A and 10B, with the white and yellowparticles travelling together in a negatively charged aggregate, and themicrocapsule 1100 can be switched between dark blue (see the left handside of FIG. 11A) and yellow (see the right hand side of FIG. 11A)states. (The dark blue state being due to light entering from theviewing surface, passing through the cyan fluid, being reflected fromthe white particles and passing back through the cyan fluid and themagenta particles.) Since the white and yellow particles are aggregatedtogether, and provide a weaker yellow than would be obtainable if theyellow particles were located between the viewer and the whiteparticles, a very short pulse of higher impulse (insufficient to invertthe positions of the magenta and white particles) may be used toseparate the white from the yellow particles to enable a better yellow(or, in the state shown on the left hand side of FIG. 11A, a betterblue) color. In the embodiment of the present invention shown in FIGS.11A-11D, the weakest color is likely to be the complement to theparticles having the lower positive charge (in FIGS. 11A-11D, these arethe yellow particles and the weak color is therefore blue).

Under mid-impulse driving conditions (see FIG. 11B), the microcapsulealso acts in a manner exactly analogous to the high impulse drivingconditions of FIG. 10B; the aggregates between the white and yellowparticles are broken, and all three species of particles travelindependently, so that the microcapsules switches between black (see theleft hand side of FIG. 11B) and white (see the right hand side of FIG.11B) states. The only difference between FIGS. 10B and 11B is that inthe latter the black state is caused by both the magenta and the yellowparticles being disposed adjacent the front electrode 1110, and lightpassing through these particles and through the cyan fluid 1108.

When the driving impulse is increased even further (see FIG. 11C), thewhite particles behave as if they were positively charged, and all threepigments migrate towards the more negative electrode, such thatsuccessive magenta, yellow and white layers are formed reading outwardlyfrom the more negative electrode cf. FIG. 8B. The resultant displayedcolors are red (see the left hand side of FIG. 11C; the color isproduced by light passing through the magenta and yellow particles,being reflected from the white particles and passing back through themagenta and yellow particles) and cyan (see the right hand side of FIG.11C; the color is produced by light passing through the cyan fluid 1108,being reflected from the white particles and passing back through thecyan fluid).

The last two colors of the microcapsule 1100 are produced by theso-called polarity reversal states shown in FIG. 11D. To produce thegreen state shown on the left hand side of FIG. 11D, one first drivesthe microcapsule with a mid-level impulse with the rear electrode 1112positive to produce the state shown on the left hand side of FIG. 11B,then reverses the rear electrode to a negative polarity and, still usinga mid-level impulse, applies the negative polarity for a periodsufficient to cause the highly charged magenta particles to move throughthe yellow and white particles until they lie adjacent the rearelectrode and the microcapsule assumes the state shown on the left handside of FIG. 11D. In this state, light entering the viewing surfacepasses through the cyan fluid and the yellow particles, is reflectedfrom the white particles (which mask the magenta particles) and passesback through the yellow particles and the cyan fluid, so that themicrocapsule displays a green color.

Similarly, the magenta state shown on the right hand side of FIG. 11D isproduced by first driving the microcapsule with a mid-level impulse withthe rear electrode 1112 negative to produce the state shown on the righthand side of FIG. 11B, then reversing the rear electrode to a positivepolarity and, still using a mid-level impulse, applying the positivepolarity for a period sufficient to cause the highly charged magentaparticles to move through the yellow and white particles until they lieadjacent the front electrode and the microcapsule assumes the stateshown on the right hand side of FIG. 11D. In this state, light enteringthe viewing surface passes through the magenta particles, is reflectedfrom the white particles (which mask the yellow particles) and passesback through the magenta particles, so that the microcapsule displays amagenta color.

Electrophoretic media of the present invention comprise a fluid and atleast the following additional components:

-   -   (a) first and second particles bearing charge of opposite        polarity; typically at least one, and normally both, of the        particles bears a polymer surface coating, although as        previously noted the possibility of controlling the        particle-particle interactions in other ways is not excluded.        For example, the microcapsule 1100 shown in FIGS. 11A-11D        comprises is negatively charged white particles and positively        charged magenta particles. The non-white particle is preferably        substantially non-scattering (i.e., light-transmissive) and of        one of the subtractive primary colors (yellow, magenta or cyan);    -   (b) a third particle that may or may not bear a polymer surface        coating (or other treatment for controlling particle-particle        interactions) that has a lower mass coverage per unit area of        the particle than the polymeric surface coatings of the first        and second particles. More generally, the Hamaker constant        and/or the interparticle spacing are adjusted by judicious        choice of the polymer coating(s) such that the particle pair        interactions, both Coulombic and attractive non-Coulombic, are        less between the particles of the first type and the particles        of the second type than between the particles of the first type        and the particles of the third type. For example, the        microcapsule 1100 shown in FIGS. 11A-11D comprises positively        charged yellow particles. The third particle is preferably        substantially non-scattering (i.e., light-transmissive) and of        one of the subtractive primary colors different from that of the        first or second pigments;    -   (c) a dye that is soluble or dispersible in the fluid and of the        third subtractive primary color; for example, the microcapsule        1100 shown in FIGS. 11A-11D comprises a cyan dye;    -   (d) at least one charge-control agent;    -   (e) a charging adjuvant; and    -   (f) a polymeric stabilizer.

In one preferred embodiment of the present invention, the first (white)particle is a silanol-functionalized scattering material such astitanium dioxide to which a polymeric material has been attached; thesecond particle is a positively charged magenta material such asdimethylquinacridone that has been coated as described below, and thethird pigment is, if cyan, a copper phthalocyanine material such asHeliogen (Registered Trade Mark) Blue D 7110 F, available from BASF,used uncoated, or, if yellow, an organic pigment such as Pigment Yellow180, again used uncoated.

The dye in this preferred embodiment is a hydrocarbon (Isopar E)-solublematerial that may be an azo dye such as Sudan I or Sudan II orderivatives thereof. Other hydrocarbon-soluble dyes such as azomethine(yellow and cyan are readily available) or other materials that arewell-known in the art may also be used as shown in the Examples below.An especially preferred cyan dye for use in the media of the presentinvention is represented by the following structure:

wherein R is a branched or unbranched hydrocarbon chain comprising atleast six carbon atoms that may be saturated or unsaturated. It may bedesirable to use a mixture of dyes, for example, mixtures of two or moredyes of the above formula having differing R groups. Use of suchmixtures may afford better solubility in the hydrocarbon fluid whilestill allowing the individual dye molecules to be purified byrecrystallization. The preparation of these dyes is described in Example5 below.

The zeta potentials of the various particle in the presence of a singleCCA (e.g., Solsperse 17000) may not be ideally arranged for switching asdescribed above. A secondary (or co-) CCA can be added to theelectrophoretic medium to adjust the zeta potentials of the variousparticles. Careful selection of the co-CCA may allow alteration of thezeta potential of one particle while leaving those of the otherparticles essentially unchanged, which allows close control of both theelectrophoretic velocities of the various particles during switching andthe inter-particle interactions. FIG. 12 is a graph showing thevariation in the zeta potentials of four particles in the presence ofSolsperse 17000 in Isopar E with the additions of small proportions ofan acidic material (Bontron E-88, available from Orient Corporation,Kenilworth, N.J., and stated by the manufacturer to be the aluminum saltof di-t-butyl salicylic acid). Addition of the acidic material moves thezeta potential of many particles (though not all) to more positivevalues. It will be seen from FIG. 12 that use of 1% of the acidicmaterial and 99% of Solsperse 17000 (based on total weight of the twomaterials) moves the zeta potential of Pigment Yellow 180 from about −5mV to about +20 mV. The addition of the same proportion of acidicmaterial changes the zeta potential of a polymer-coated white particle(prepared as described in U.S. Pat. No. 7,002,728) from about −45 mV toabout −20 mV. However, the zeta potential of the magenta pigment islargely unchanged by addition of the aluminum salt. Whether or not thezeta potential of a particular particle is changed by a Lewis acidicmaterial like the aluminum salt will depend upon the details of thesurface chemistry of the particle.

The addition of a basic co-CCA (e.g., OLOA 371, available from ChevronCorporation, Sam Ramon, Calif.) will move the zeta potentials of variouspigments to more negative values.

The following Examples are now given, though by way of illustrationonly, to shows details of particularly preferred materials, processes,conditions and techniques used to prepare the media and electrophoreticdisplays of the present invention.

Example 1

This Example illustrates the preparation of a two particle colored fluidelectrophoretic display of the type illustrated in FIG. 8 of theaccompanying drawings.

Part A: Preparation of a Magenta Pigment Dispersion

Ink Jet Magenta E 02 VP2621, available from Clariant, Basel,Switzerland, (15 g) was dispersed in toluene. The resultant dispersionwas transferred to a 500 mL round-bottomed flask and the flask degassedwith nitrogen. The reaction mixture was then brought to 42° C., and,upon temperature equilibration, 4-vinylbenzylchloride was added and thereaction was allowed to stir at 42° C. under nitrogen overnight. Theresulting reaction mixture was allowed to cool to room temperature andthen centrifuged to isolate the functionalized pigment. The centrifugecake was washed with toluene (3×250 mL) to produce 14.76 g of a magentapigment functionalized with a vinyl group to which a polymeric chaincould be attached.

The dried pigment thus was dispersed in toluene with sonication androlled on roll mill, and the resultant dispersion transferred to atwo-neck 500 mL round-bottomed flask equipped with large magnetic stirbar and the flask was placed into a preheated silicone oil bath held at65° C. Lauryl methacrylate was added to the flask, a Vigreux distillingcolumn was attached for use as an air condenser, and the second neck ofthe flask was closed with a rubber septum. The system was purged withnitrogen for at least one hour, and then a solution of AIBN(2,2′-azobis(2-methylpropionitrile)) in toluene was syringed into thereaction flask all at once. The reaction mixture was stirred vigorouslyat 65° C. overnight, then poured into a 1 L plastic centrifuge bottle,diluted with toluene and centrifuged at 4500 RPM for 30 minutes. Thecentrifuge cake was washed once with toluene and the mixture was againcentrifuged at 4500 RPM for 30 minutes. The supernatant was decanted andthe resultant pigment was dried in a 70° C. vacuum oven overnight, thenground with a mortar and pestle, and dispersed in Isopar E to form a 20weight % dispersion, which was sonicated and rolled on a roll mill forat least 24 hours (or longer if) desired. The resultant dispersion wasfiltered through fabric mesh to remove any large particles, a sampleremoved and its solids content measured.

Part B: Preparation of Internal Phase

The magenta pigment dispersion prepared in Part A above (13.92 g of a14% w/w dispersion in Isopar E) was combined with 83.07 g of a 60% w/wIsopar E dispersion of titanium dioxide (polymer coated as described inthe aforementioned U.S. Pat. No. 7,002,728), 7.76 g of a 20% w/wsolution of Solsperse 17000 in Isopar E, a 15% w/w solution ofpoly(isobutylene) of molecular weight 1,270,000 in Isopar E (thispoly(isobutylene) acts as an image stabilizer; see U.S. Pat. No.7,170,670), 0.575 g of Sudan 1 of the formula:

(available from Acros Organics, New Jersey) and 5.82 g of Isopar E. Theresultant mixture was dispersed overnight on a mechanical roller toproduce an internal phase ready for encapsulation and having aconductivity of 304.7 pS/cm.

Part C: Microencapsulation

The internal phase prepared in Part B was encapsulated following theprocedure described in U.S. Pat. No. 7,002,728. The resultantencapsulated material was isolated by sedimentation, washed withdeionized water, and size-separated by sieving. Capsule size analysisusing a Coulter Multisizer showed that the resulting capsules had a meansize of 40 μm and more than 85 percent of the total capsule volume wasin capsules having the desired size of between 20 and 60 μm.

Part D: Preparation of Display

The sieved capsules produced in Part C above were adjusted to pH 9 withammonium hydroxide solution and excess water removed. The capsules werethen concentrated and the supernatant liquid discarded. The concentratedcapsules were mixed with an aqueous polyurethane binder (prepared in amanner similar to that described in U. S. Patent Application PublicationNo. 2005/0124751) at a ratio of 1 part by weight binder to 15 parts byweight of capsules following which Triton X-100 surfactant andhydroxypropylmethylcellulose were added and mixed thoroughly to providea slurry.

The capsule slurry thus prepared was coated onto the indium tin oxide(ITO) coated surface of a poly(ethylene terephthalate) (PET)/ITO film of125 μm thickness using a bar coater, and the coated film dried at 60° C.Separately, a layer of polyurethane adhesive doped withtetraethylammonium hexafluorophosphate as a conductive dopant was coatedonto a release sheet, and the resultant PET film/adhesive sub-assemblywas laminated on top of the coated capsules as described in theabove-mentioned U.S. Pat. No. 7,002,728. The release sheet was removedand the resultant multilayer structure was laminated onto a graphiterear electrode to produce an experimental single-pixel displaycomprising, in order from its viewing surface, the PET film, a layer ofITO, a capsule layer, a lamination adhesive layer, and the graphite rearelectrode.

Part E: Electro-Optic Tests

The resulting displays were switched using a square-wave AC waveformapplied to the graphite rear electrode (while the front ITO electrodewas grounded) of ±30V and 50 Hz that was offset from zero as specifiedbelow (for example, a 5V offset would provide 50 Hz square waveoscillations of +35/−25V). Table 1 below shows the reflectivities (inpercent) at various wavelengths obtained for the indicated color statesof the display.

TABLE 1 Red Magenta White Yellow 450 nm 13.4 16.8 31.5 13.5 550 nm 8.89.7 44.9 35.2 650 nm 60.8 55.0 60 54.4

The DC offset for red/white switching was ±10V. In this case, the whiteand magenta pigments move though the yellow, dyed fluid. The red stateresults from viewing of the magenta (green-absorbing) pigment and theyellow (blue-absorbing) dye against the white background. The DC offsetfor magenta/yellow switching was ±60V. The yellow color was obtained asthe white pigment moved away from the viewing side of the displaytowards the negatively-charged rear electrode, as described above withreference to FIG. 8.

In summary, at low applied fields the white pigment behaves as though itwere negatively charged, being driven to the front electrode when therear electrode is at a relatively low, negative voltage, and providinghigh reflectivity at 450 nm (a wavelength absorbed by the dye); at morenegative applied rear electrode voltages, the white pigment movestowards the rear electrode, behaving as though it were positivelycharged, exposing the dye and reducing the reflectivity at 450 nm.

Example 2

This Example illustrates the preparation of a three particle undyedfluid electrophoretic display of the type illustrated in FIGS. 10A and10B of the accompanying drawings.

Part A: Preparation of a Yellow Pigment Dispersion

A yellow pigment, Novoperm Yellow P-HG, available from Clariant, Basel,Switzerland was combined with Isopar E and a solution of Solsperse 17000in Isopar E, and the mixture was dispersed by attriting vigorously usinga Szegvari Attritor (Registered Trade Mark) type 01-HD, size 01 at 650rpm with 0.4-0.6 mm glass beads for 1 hour to afford a yellow pigmentdispersion.

Part B: Preparation of a White Pigment Dispersion

Titanium dioxide was treated with a silane as described in theaforementioned U.S. Pat. No. 7,002,728. The resultant silane-treatedwhite pigment was treated with a monomer and a polymerization initiatoras described in U.S. Patent Application Publication No. 2011/0012825 toproduce a polymer coated white pigment, which was combined with Isopar Eto yield a white pigment dispersion.

Part C: Preparation of a Black Pigment Dispersion

A black pigment (BK444 or BK20C920, available from Shepherd ColorCompany, Cincinnati, Ohio) was milled in water to a particle size ofabout 300 nm for BK444 and 500 nm for BK20C920. The milled pigment wassurface-functionalized usingN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediaminebishydrochloride (available from United Chemical Technologies) in amanner analogous to that described in U.S. Pat. No. 6,822,782.Thermogravimetric analysis (TGA) indicated the presence of 4-10% ofvolatile (organic) material for BK444 and 1.1-1.3% of volatile materialfor BK20C920. A lauryl methacrylate coating was then provided on thepigment as described in U.S. Pat. No. 6,822,782. The final pigmentshowed 15-25% volatile material by TGA for BK444 and 4-6% volatilematerial for BK20C920.

Part D: Preparation of Electrophoretic Medium

The yellow pigment dispersion prepared in Part A above (1.91 g) wascombined with the black dispersion made from BK444 in Part C above (0.92g), the white dispersion prepared in Part B above (4.95 g), aluminum3,5-di-tert-butylsalicylate, available from Esprix Technologies,Sarasota, Fla., (0.1 g of a 1% w/w solution in Isopar E), the samepoly(isobutylene) as in Example 1 above, 0.46 g of a 15% w/w solution inIsopar E) and 1.66 g of Isopar E. The resultant mixture was sonicatedand warmed to 42° C. for 30 minutes to produce an electrophoretic mediumcomprising the three pigments in a hydrocarbon fluid having aconductivity of 240 pS/cm.

Part E: Electro-Optic Tests

Cell (a):

A parallel-plate cell was prepared consisting of two 50 mm×55 mm glassplates each coated with a transparent, conductive coating of ITO. Theelectrophoretic medium prepared in Part D above (15 μL) was dispensedonto the ITO-coated face of the lower glass plate and then the upperglass plate was placed over the electrophoretic medium so that the ITOcoating was in contact with the fluid. Electrical connections were thenmade to the cell by use of conductive copper tape affixed to the ITOcoated sides of both the top and bottom glass plates.

Cell (b):

Cell (b) was prepared as described above for Cell (a) except that theconductive ITO coatings on each of the glass plates were blocked byapplication of a polymer overcoat (a solution of poly(methylmethacrylate) (PMMA) in acetone was bar-coated using a #7 Mayer rod togive a dry coating approximately 0.5 μm in thickness).

Cells (a) and (b) were electrically driven with a waveform consisting ofa square wave of 10 Hz frequency at voltages of ±30, 15, and 7.5 Vapplied to the lower electrode while the upper electrode was grounded,using the duty cycle sequence shown in Table 2 below, preceded by a setof shake-up pulse trains at ±30V, 10 Hz, 6×1 sec. duration, with dutycycles of 0.05, 0.1, 0.2, 0.4, 0.8 and 1.

TABLE 2 Time (sec) Duty Cycle 0 0.05 1 .1 2 .2 3 .4 4 .8 5 1 6 .8 7 .4 8.2 9 .1 10 .05

Reflection spectra were acquired as Cells (a) and (b) were electricallydriven, giving the results shown in FIGS. 13A and 13B respectively. Ascan be seen from these Figures, in Cell (a), in which the electrodeswere unblocked, the cell was capable of rendering black, white andyellow states (in FIGS. 13A and 13B, more positive values of b* indicateincreasing yellow coloration, while more positive values of L* indicateincreasing lightness). In Cell (b), in contrast, the electrodes wereblocked and minimal current passed, so no white state was seen (there isno state with high L* and low b*); the cell switched simply betweenblack and yellow states.

Example 3

This Example illustrates the preparation of a further three particleundyed fluid electrophoretic display of the type illustrated in FIGS.10A and 10B of the accompanying drawings.

An internal phase was prepared from the following components (byweight):

-   -   White pigment (from Example 2, Part B): 29.7%    -   Black pigment (from Example 2, Part C): 6.0%    -   Yellow pigment (from Example 2, Part A): 3.0%    -   Solsperse 17000: 2.0%    -   Aluminum di-t-butyl salicylate: 0%    -   Poly(isobutylene) (as in Examples 1 and 2): 1.05%

The internal phase so prepared was then encapsulated as described inU.S. Pat. No. 7,002,728. The resultant capsules were isolated bysedimentation, washed with deionized water, and size separated bysieving. Capsule size analysis using a Coulter Multisizer showed thatthe resulting capsules had a mean size of 40 μm and more than 85 percentof the total capsule volume was in capsules having the desired size ofbetween 20 and 60 μm. The capsules were then converted to single pixelexperimental displays with graphite rear electrodes as described inExample 1, Part D above.

A display so constructed was driven using the waveform as shown in FIG.14 (the rear electrode voltage relative to a grounded front electrode isshown). The waveform consisted of a 1 second pulse of −15V, followed bya 1 second pulse of +15V, followed by a test pulse of −15V that variedin length from 50 ms to 400 ms in increments of 50 ms. Also shown inFIG. 14 are the L* and b* values measured as the display was driven.During the test pulse the display switched from black, via yellow, towhite (as indicated by arrows 906 and 902 in FIG. 9), while during the+15V pulses, the display switched from white to black (as indicated byarrow 904 in FIG. 9).

Example 4

This Example illustrates the preparation of a three particle dyed fluidelectrophoretic display of the type illustrated in FIGS. 11A-11D of theaccompanying drawings.

Part A: Preparation of a Cyan Pigment Dispersion

A cyan pigment, Irgalite Blue GLVO, available from BASF, Ludwigshafen,Germany was combined with Isopar E and a solution of Solsperse 17000,and the resultant mixture was dispersed by attriting vigorously at 650rpm with 0.4-0.6 mm glass beads for 1 hour to afford a cyan pigmentdispersion.

Part B: Preparation of Electrophoretic Medium

An electrophoretic medium was prepared from the following components (byweight):

-   -   White pigment (from Example 1, Part B): 29.7%    -   Magenta pigment (from Example 1, Part A): 1.3%    -   Cyan pigment (from Part A above): 0.75%    -   Sudan Yellow dye 0.75%    -   Solsperse 17000: 2.0%    -   Aluminum di-t-butyl salicylate: 0.02%    -   Poly(isobutylene) (as in Examples 1-3): 1.05%

The resultant fluid was loaded into Cell (a) described in Example 2above, and addressed with a waveform consisting of a square wave of 30Hz frequency at voltages of ±10, 15, 20 and 40 V applied to the rearelectrode while the front electrode was grounded, using the duty cyclesequence shown above in Table 2.

The light reflected from the test cell was analyzedspectrophotometrically, and the closest approaches in CIE L*a*b* to theSNAP color standard were recorded. These values (a* and b*) are shown inTable 3 below. It can be seen that the electrophoretic fluid was able todistinguish all the primary colors (CMYRGBKW).

TABLE 3 L* a* b* C 49.5 −23.4 −15.7 G 61.6 −18.4 1.9 Y 43.9 −5 20 R 27.622.9 12.2 M 33.6 30.8 −7.9 B 38.1 6.5 −21.4 K 28.1 1.4 3.3 W 63.8 −9 4.3

Example 5

This Example illustrates the preparation of a group of cyan dyes usefulin the electrophoretic media and displays of the present invention.

Part A: Preparation of a First Cyan Dye

This Part of this Example illustrates the preparation of a cyan dye bythe reaction:

where R represents a C₁₂H²⁵ alkyl group. The reaction is adapted fromExample 3 in U.S. Pat. No. 5,122,611.

To a two-neck 500 mL round-bottomed flask equipped with reflux condenserand magnetic stir bar were added4-bromo-N-dodecyl-1-hydroxy-2-naphthamide, dichloromethane (DCM) andethanol. To the resultant reaction mixture were addedN,N-diethyl-p-phenylenediamine sulfate salt in deionized water, andpotassium carbonate in deionized water followed by ammonium persulfatein deionized water. The reaction mixture was stirred at room temperaturefor 30 minutes, and then poured into a large separatory funnel andseparated. The aqueous layer was extracted with DCM, and the organiclayers were washed with deionized water. The resulting organic phase wasconcentrated under reduced pressure, and the crude material produced waspurified via recrystallization with DCM and methanol.

The dye had λ_(max) of 648 nm in Isopar E solution, ε=28,100 Lmol⁻¹cm⁻¹.The solubility of the dye in Isopar E at 4° C. was 1.2 wt %.

Part B: Preparation of a Second Cyan Dye

This Part of this Example illustrates the preparation of a cyan dye bythe three-step reaction sequence:

The first step of this reaction sequence is adapted from Huang, Y.;Luedtke, R. R.; Freeman, R. A.; Wu, L.; Mach, R. H. J. Med. Chem. 2001,44, 1815-1826, and the third step from U.S. Pat. No. 5,122,611.

Step 1:

To a 1 L round-bottomed flask equipped with an overhead stirrer wereadded 1-hydroxy-2-naphthoic acid, pyridinium bromide perbromide andacetic acid. The resultant reaction mixture was stirred at roomtemperature overnight, then filtered, and the resulting solid was washedwith deionized water, dried under vacuum and used without furtherpurification.

Step 2:

To a 250 mL round-bottomed flask were added4-bromo-1-hydroxy-2-naphthoic acid and N,N-dimethylformamide (DMF). Oncethe acid had dissolved, 1-hydroxybenzotriazole hydrate andN-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride were addedto the flask. Finally, oleylamine was added to the flask via a syringe.The resultant reaction mixture was stirred at room temperature for 5days, then poured into deionized water and extracted withdichloromethane (DCM, 3×100 mL aliquots). The organic phases werecombined and washed with 10 wt % hydrochloric acid solution (4×100 mL).A solid formed and was filtered away from the organic layer. The organiclayer was filtered through a silica plug and the product wasconcentrated under reduced pressure (48% yield).

Step 3:

To a two-necked 500 mL round-bottomed flask equipped with a refluxcondenser and a magnetic stir bar were added4-bromo-1-hydroxy-N-oleyl-2-naphthamide, DCM and ethanol. To theresultant reaction mixture were added N,N-diethyl-p-phenylenediaminesulfate salt in deionized water, and potassium carbonate in deionizedwater, followed by ammonium persulfate in deionized water. The reactionmixture was stirred at room temperature for 30 minutes, then poured intoa large separatory funnel and separated. The aqueous layer was extractedwith DCM, and the combined organic layers washed with deionized water.The resulting organic phase was concentrated under reduced pressure togive a crude material, which was purified via silica gel chromatographywith DCM as the eluent.

The resulting dye had λ_(max) of 622 nm in Isopar E solution, ε=25,800Lmol⁻¹cm⁻¹. The solubility of the dye in Isopar E at 4° C. was 3.9 wt %.

Example 6

This Example illustrates the preparation of a three particle, dyed fluidelectrophoretic display of the type illustrated in FIGS. 11A-11D of theaccompanying drawings.

Part A: Preparation of a Yellow Pigment Dispersion

A yellow pigment, Novoperm Yellow P-HG, available from Clariant, Basel,Switzerland was combined with Isopar E and a solution of Solsperse 17000in Isopar E, and the mixture was dispersed by attriting vigorously usinga Szegvari Attritor type 01-HD, size 01 at 650 rpm with 0.4-0.6 mm glassbeads for 1 hour to afford a yellow pigment dispersion.

Part B: Preparation of a Magenta Pigment Dispersion

Ink Jet Magenta E 02 VP2621, available from Clariant, Basel,Switzerland, was dispersed as a 10% w/w in toluene. The pigmentdispersion was transferred to a 500 mL round-bottomed flask and theflask degassed with nitrogen, and the solution brought to 42° C. Uponreaching this temperature, 4-vinylbenzylchloride was added and theresultant reaction mixture was stirred at 42° C. under nitrogenovernight. The resulting product was allowed to cool to room temperatureand centrifuged to isolate the functionalized pigment. The centrifugecake was washed with toluene (3×250 mL) to produce the functionalizedmagenta pigment.

The magenta pigment thus prepared was coated with a lauryl methacrylatecoating as described in the abovementioned U.S. Pat. No. 7,002,728. Thefinal pigment was then combined with Isopar E to produce a magentapigment dispersion, which was filtered through a 200 micrometer meshfilm and its solids content was determined to be 15.9%.

Part C: Preparation of a White Pigment Dispersion

A titania dispersion was produced as in Example 2, Part B above.

Part D: Preparation of an Electrophoretic Medium and Electro-Optic Tests

The yellow pigment dispersion prepared in Part A above (0.65 g), themagenta dispersion prepared in Part B above (0.83 g), the whitedispersion prepared in Part C above (3.22 g), the cyan dye prepared inExample 5, Part A above (0.10 g), aluminum 3,5-di-tert-butylsalicylate(0.07 g of a 1% w/w solution in Isopar E), poly(isobutylene) ofmolecular weight 600,000 (0.31 g of a 15% w/w solution in Isopar E) and1.26 g of additional Isopar E were mixed. The resultant mixture wassonicated and warmed to 42° C. for 30 minutes to produce anelectrophoretic medium having a conductivity of 74 pS/cm.

This electrophoretic medium fluid was loaded into a first test celldescribed in Example 2 above. Reflection spectra were obtained as therear electrode was driven with the waveforms shown in FIGS. 15A-15Jwhile the front electrode was grounded. FIGS. 16A-16J show the opticalstates (plotted as graphs of L*, a* and b* against time) achieved usingthe waveforms in FIGS. 15A-15J respectively. The sample rate of thespectrometer was 20 Hz, so the optical transients during the resetpulses at the beginning of each waveform were not recorded.

In FIGS. 15A-15F, the waveforms used consisting of a series of rapidlyoscillating reset pulses of ±30V or ±15V followed by 1.5 seconds ofdriving with a constant rear electrode of −30V (FIG. 15A), −15V (FIG.15B). −7.5 V (FIG. 15C), +30V (FIG. 15D), +15V (FIG. 15E) or +7.5V (FIG.15F). The waveforms shown in FIGS. 15G-15J are of a different typecomprising the same series of rapidly oscillating reset pulses of ±30Vor ±15V, but using a driving portion of the waveform in which the rearelectrode voltage alternates between positive and negative voltages,with rests at zero voltage between the positive and negative impulses.The pulses are ±30V (FIG. 15G) and ±15V (FIG. 15H). The waveforms inFIGS. 15I and 15J are essentially inverted forms of the waveforms inFIGS. 15G and 15H respectively, in the sense that in FIGS. 15G and 15H,the driving sequence ispositive-zero-negative-zero-positive-zero-negative-zero etc., whereas inFIGS. 15I and 15J the driving sequence isnegative-zero-positive-zero-negative-zero-positive-zero etc.

From FIGS. 16A-16C, it will be seen that the results of the waveforms ofFIGS. 15A-15C is that b* starts positive in all three cases, and movesto negative values as the pulse continues, while a* remains fairlyconstant; thus, the display changes from a (greenish) yellow tint to acyan tint, crossing zero at a cyan-tinted white. The waveform of FIG.15C produces an almost white state. These results are consistent withthe switching mechanism proposed above with reference to the right handsides of FIGS. 11A-11C. In contrast, with the positive driving waveformsof FIGS. 15D-15F, FIGS. 16D-16F show display switching from a dark bluestate (b* about −10) to a red state (a* +22; b* about +10), and thewaveform of FIG. 15F producing an almost black state. These results areconsistent with the switching mechanism proposed above with reference tothe left hand sides of FIGS. 11A-11C.

FIGS. 16G and 16H show that the reversing waveforms of FIGS. 15G and 15Hcause the display to oscillate between magenta (rear electrode positive)and green states (rear electrode negative); the best magenta is obtainedusing the waveform of FIG. 15H. Similarly, FIGS. 16I and 16J show thatthe reversing waveforms of FIGS. 15I and 15J cause the display tooscillate between green (rear electrode negative) and red/magenta states(rear electrode positive); the best green is obtained using the waveformof FIG. 15J. The difference between the waveforms of FIGS. 15G/15H andthose of FIGS. 15I/15J, each of which alternates between positive andnegative driving impulses, is that the reset train begins and ends withnegative pulses in waveforms the waveforms of FIGS. 15G/15H and withpositive pulses in those of FIGS. 15I/15J. Thus, the starting point hasa net negative impulse in the waveforms of FIGS. 15G/15H and a netpositive impulse in those of FIGS. 15I/15J. The starting net negativeimpulse favors magenta over green, while the starting net positiveimpulse favors green over magenta.

FIG. 17 is a plot on an a*/b* plane of all the colors obtained by thewaveforms of FIGS. 15A-15J, and from the Figure it can be seen that allthe primary colors are afforded by this electrophoretic display of theinvention.

Example 7

This Example illustrates the preparation of a second three particle,dyed fluid electrophoretic display of the type illustrated in FIGS.11A-11D of the accompanying drawings.

Part A: Preparation of a Cyan Pigment Dispersion

A cyan pigment, Hostaperm Blue BT-617-D, available from Clariant, Basel,Switzerland (26 g) was mixed with Isopar E (70 g) and a solution ofSolsperse 17000 (70 g of a 20% w/w solution in Isopar E) and theresultant mixture was dispersed by attriting vigorously at 650 rpm with0.4-0.6 mm glass beads for 1 hour to afford a cyan pigment dispersion.

Part B: Preparation of Electrophoretic Medium and Electro-Optic Tests

An electrophoretic medium was prepared from the following components (byweight):

-   -   White pigment (from Example 3, Part C): 29.7%    -   Magenta pigment (from Example 1, Part A): 2.1%    -   Magenta pigment (from Part A above): 0.75%    -   Automate Yellow dye (Dow Chemical) 0.75%    -   Solsperse 17000: 0.785%    -   Aluminum di-t-butyl salicylate: 0.01%    -   Poly(isobutylene) (as in Examples 1-3): 1.05%

The resultant fluid was placed in Cell (a) described in Example 2 above,and driven with the waveforms shown in FIGS. 15A-15J. FIG. 18 is a plotin the a*b* plane, similar to that of FIG. 17, of all the colorsobtained. From FIG. 18, it will be seen that all the primary colorsexcept red are provided by this electrophoretic medium.

Example 8

This Example illustrates the preparation of a third three particle, dyedfluid electrophoretic display of the type illustrated in FIGS. 11A-11Dof the accompanying drawings.

An internal phase was prepared from the following components (byweight):

-   -   White pigment (from Example 3, Part C): 29.7%    -   Magenta pigment (from Example 1, Part A): 3.0%    -   Yellow pigment (from Example 2, Part A): 2.5%    -   Cyan due (from Example 6, Part A) 1.5%    -   Solsperse 17000: 1.24%    -   Aluminum di-t-butyl salicylate: 0.01%    -   Poly(isobutylene) (as in Examples 1-3): 1.05%

The internal phase thus prepared was encapsulated following theprocedure described in U.S. Pat. No. 7,002,728. The resultantencapsulated material was isolated by sedimentation, washed withdeionized water, and size-separated by sieving. Capsule size analysisusing a Coulter Multisizer showed that the resulting capsules had a meansize of 74 μm and more than 85 percent of the total capsule volume wasin capsules having the desired size of between 50 and 100 μm. Thecapsules were then converted to experimental single pixel displays inthe same manner as in Example 1, Part D above.

These displays were then driven with the waveforms shown in FIGS.15A-15J. FIG. 19 is a plot in the a*b* plane, similar to those of FIGS.17 and 18, of all the colors obtained. From FIG. 19, it will be seenthat all the primary colors are provided by this electrophoretic medium.

Example 9—Waveform Optimization for Spot Color

Subsequent to the experiments described in Example 3 above, it wasdiscovered that the type of waveform shown in FIG. 14 is not in fact theoptimum waveform for obtaining good spot color in the type ofthree-particle black/white/spot color electrophoretic medium shown inFIGS. 10A and 10B. (The spot color is shown in FIGS. 10A and 10B asyellow, and the same spot color will be assumed in the followingdiscussion, but this is for purposes of illustration only and any spotcolor other than white or black may of course be used.) It has beenfound that better saturation (i.e., increased b* value for yellow spotcolor, and increased a* value for some other spot colors) can beachieved by using a square wave with appropriately chosen frequency andduty cycle.

As illustrated in FIGS. 10A and 10B, the positively charged blackpigment moves to the viewing (upper) surface of the display when therear electrode 1012 is positive relative to the front electrode 1010 andto the back of the display when the rear electrode 1012 is negativevoltage relative to the front electrode 1010. On the other hand, thenegatively charged white pigment moves to the viewing surface of thedisplay when the rear electrode is negative and to the back of thedisplay when the rear electrode is positive. The third (yellow) pigmentis initially negatively charged and under low impulse, when the rearelectrode is negative, the yellow pigment at first moves to the viewingsurface of the display (FIG. 10A, right side—the display looks yellow),but if the voltage is applied for a long enough time to provide a highaddressing impulse, the yellow colored pigment disappears behind thewhite pigment and the display changes from yellow to white (FIG. 10B,right side).

These color changes are illustrated in FIG. 14. Consider the first cycleshown in FIG. 14. Initially the display is in its black state (FIG. 10A,left side). When a −15V pulse is applied, the black pigment moves to therear surface and the white and yellow pigments move to the viewingsurface (FIG. 10A, right side). Initially b* and L* increase as thedisplay turns yellow. After some time, b* (“yellowness”) reaches itsmaximum, and then decreases, while L* keeps increasing, as a highaddressing impulse is reached and the display turns white (FIG. 10B,right side).

It has found that a very simple waveform using alternating negative andpositive pulses can achieve a maximum b* value higher than thatachievable with a single positive-negative cycle, as shown in FIG. 14.This waveform is illustrated in FIG. 20, from which it will be seen thatthis waveform comprises a series of short (about 0.5 second) negativepulses separated by longer (1 second) positive pulses and terminatingwith one of the negative pulses. The optimum duration of the positiveand negative transitions is somewhat dependent upon the composition ofthe electrophoretic medium and it is usually several hundredmilliseconds long. The key factor in determining how long the negativepulse needs to be is how long one can apply a negative drive (fromblack) before the yellow color starts decreasing. The positive pulseshould be longer than the negative pulse, and needs to be long enough todrive the display back to the black extreme optical state. As shown inFIG. 20, the waveform used increases b* to about 55 as compared withabout 41 for the waveform shown in FIG. 14.

FIG. 21 shows a plot of L* against b* for the first cycle shown in FIG.14 and for the waveform of FIG. 20. It will be seen that the extremeyellow state of the FIG. 20 waveform (indicated as “Y” in FIG. 21 has asubstantially higher b* value than that of the FIG. 14 waveform. Onepossible explanation for this improvement in b* is that some of theyellow pigment starts “turning around” (i.e., moving away from theviewing surface of the display) before the best yellow state isachieved. By briefly reversing the polarity of the driving pulse, thecharge on the yellow pigment is reset and when the voltage switches backto negative, the yellow pigment keeps traveling to the viewing surfaceof the display and most, if not all, reaches this surface, resulting ina better yellow state and improved maximum b*.

While the foregoing discussion has focused on an improved yellow state,the type of waveform shown in FIG. 20 has other advantages. By varyingthe length of the drive pulses, it has been possible to achieve improvedintermediate optical states. As shown in FIG. 21, “gray” states, withvariable L*, and very low b*, are obtained by moving along the W-K pathin FIG. 21, while “Y-gray” states, with variable L* and variable b*, areobtained by moving along the K—Y-W path in FIG. 21.

The type of waveform shown in FIG. 20 can be incorporated into anoverall DC-balanced drive scheme (see the aforementioned MEDEODapplications for the importance of maintaining overall DC balance in adrive scheme for bistable electro-optic displays) in a variety of ways,for example:

-   -   (a) Self-balanced transitions: as described in U.S. Pat. No.        7,119,772, in such a drive scheme each waveform defining a        transition between two optical states has zero net impulse.        Since the optimum FIG. 20 type waveform for a transition to        yellow may have a net impulse, this net impulse must be        counterbalanced, for example by a pre-pulse having an opposite        impulse. The waveform shown in FIG. 20 for transition to yellow        has a net positive impulse so the overall transition to yellow        would first have an equal negative impulse (toward white) prior        to the FIG. 20 waveform; and    -   (b) Round-trip DC balance: as described in U.S. Pat. No.        7,012,600, in many prior art drive schemes, individual waveforms        are not DC balanced. Instead the entire drive scheme is designed        such that every closed loop of transitions (i.e., each set of        transitions beginning and ending on the same gray level) has        zero net impulse. To achieve this, each optical state is        assigned an “impulse potential” and the net impulse of the        waveform used in any transition between two different optical        states must equal he difference in impulse potentials between        those two optical states. FIG. 22 illustrates such a drive        scheme. The ellipses represent optical states with assigned        impulse potentials. The directed arrows show the net impulse of        a waveform between the two optical states represented at the end        of the arrows; this impulse must be equal to the difference in        impulse potentials between these two optical states. A FIG. 20        type yellow transition can be incorporated into such a drive        scheme in many ways. For example, a transition to yellow can be        notionally considered as a two part transition, one from the        current state to black and then one from black to yellow. Since        the FIG. 20 type yellow waveform has a net positive impulse, it        can be considered part of the first transition to black (which        also has a net positive impulse). Then the second part of the        waveform is a negative set pulse to yellow which is equal to the        difference of the black and chosen yellow impulse potential.        FIGS. 23 and 24 illustrate this approach. FIG. 23 shows a simple        square wave drive and impulse potentials. The area highlighted        shows a black-to-yellow transition. The FIG. 20 type yellow part        of the waveform can be considered to be a component of the        white-to-black part of this of this transition as shown in FIG.        24, which shows the actual voltage pulses used in the        black-to-yellow transition.

It has been found that the optimum FIG. 20 type waveform depends uponvoltage in a manner which may differ from batch to batch ofelectrophoretic material even though the batches have the same nominalcomposition. FIG. 25 shows an example of a batch in which the optimalvoltage used for driving to yellow is in the range of 20-29 V, asopposed to the 15 V used above.

One of the factors limiting the performance of the type of display shownin FIGS. 10A and 10B is duration of the waveform required from black oryellow to white. As shown in FIG. 14, even without any reverse polaritypulses, it may take a full two seconds to transition from black towhite. This is even more pronounced when the waveform includes periodsof reverse polarity and/or when higher drive voltages are used. Tocombat these performance limitations, a “picket fence” type of waveformmay be used, as shown in FIG. 26, which shows four (simplified) examplesof this type of waveform. From top to bottom, FIG. 26 illustrates anoriginal waveform, a waveform in which the negative pulse has beenextended, a waveform in which periods of zero voltage have beeninserted, and a waveform in which periods of reverse (positive) voltagehave been added. Adding small intervals of zero, or positive, voltage tothe waveform in this manner allows more rapid removal of the yellow fromthe white state, thus enabling shorter waveforms and/or improves thewhite state by reducing the amount of yellow. FIG. 27 shows theimprovement in L* and b* values of the white optical state which can beachieved by use of picket fence waveforms. In FIG. 27, picketGapSignrepresents which type of picket fence was used; a value of 1 meansextending the drive, a value of 0 means adding periods of zero voltage,and a value of −1 means adding reverse (positive) voltage. TotalGapTimerepresents the total amount of drive time added in this way.

It will be apparent to those skilled in the technology ofelectrophoretic displays that use of the waveforms discussed above maycause a significant amount of flashing to be visible during transitions.Such flashing can be reduced by using different waveforms for two ormore sub-populations of pixels such that some fraction of the pixels arein a light optical state while some other fraction of pixels are in adark optical state; the average optical state of such a display viewedfrom a distance will then be a slowly varying gray. This flash reductiontechnique is most effective when applied to a clearing signal havingdrive pulses of both polarities since all pixels are experiencing anequal duty cycle of a periodic drive to black and drive to white and arethus easily divided into groups using differing waveforms. Thesetechniques have been previously disclosed (for black and whiteelectrophoretic displays) in some of the aforementioned MEDEODapplications.

From the foregoing, it will be seen that the present invention canprovide full color displays capable of rendering all the primary colorsover the entire area of the display. If desired, areal modulation may beused, in addition to the color modulation provided by the presentinvention, to enable the display to show a full range of saturation ineach color. The invention can also provide displays capable of producingspot color over the entire area of the display.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

The invention claimed is:
 1. An electrophoretic medium comprising afluid and first, second, and third species of particles disposed in thefluid, wherein the second and third species of particles bear charges ofopposite polarities and the first species of particles bears a charge ofthe same polarity as the third species of particles, and wherein thecolors of the three species are different, wherein when theelectrophoretic medium is addressed with a first impulse in a firstdirection, the first species of particles move in a first directionrelative to an electric field of the first impulse, and a viewingsurface of the electrophoretic medium displays the color of the thirdspecies of particle, wherein when the electrophoretic medium isaddressed with a second impulse larger than the first impulse and havingthe same polarity as the first impulse, the first species of particlesmove in a second direction that is opposite to the first direction, andthe viewing surface displays a mixture of the first and third particles,and wherein when the electrophoretic medium is addressed with a thirdimpulse greater than the second impulse, but having the same polarity asthe first and second impulses, the viewing surface displays a mixture ofthe first, second and third particles.
 2. The electrophoretic medium ofclaim 1, wherein the fluid is dyed a color.
 3. The electrophoreticmedium of claim 2, wherein one of the dyed fluid and the first, second,and third species of particles has one of the additive primary colorsand another of the dyed fluid and the first, second, and third speciesof particles has the complementary subtractive primary color.
 4. Theelectrophoretic medium of claim 3, wherein the first species ofparticles is white, and the dyed fluid or the second species ofparticles has an additive primary colors and the other of the dyed fluidand the second species of particles has a subtractive primary colorcomplementary to the additive primary color.
 5. The electrophoreticmedium according to claim 1, wherein the fluid is uncolored.
 6. Theelectrophoretic medium according to claim 5, wherein the second andthird species of particles are white and black.
 7. The electrophoreticmedium according to claim 1, wherein the first, second, or third speciesof particles is white and the colors of the other two species ofparticles and the fluid are selected from yellow, cyan and magenta, inany order.
 8. The electrophoretic medium according to claim 1, whereinthe particles and the fluid are confined within a plurality of capsulesor microcells.
 9. The electrophoretic medium according to claim 1,wherein the particles and the fluid are present as a plurality ofdiscrete droplets surrounded by a continuous phase comprising apolymeric material.
 10. An electronic book reader, portable computer,tablet computer, cellular telephone, smart card, sign, watch, shelflabel or flash drive incorporating the electrophoretic display accordingto claim 1.